Her first single, Because We Want To, was a feelgood anthem about (little) girl power, and teeny rebellion. She might as well have been called Junior Spice. Piper, then simply known as Billie, was a management construct. Virgin Records thought of the concept of Billie well before they met her. Wouldn't it be great if they could find a sweet young thing with attitude; a heroine for a newly exploited tweeny market of eight-to-11-year-olds, with crossover appeal for the early teens. As with so many manufactured pop stars, her "character" was finely nuanced: Billie wasn't projected as a true innocent, and certainly not as a precocious slut; she was an innocent affecting experience. To an extent, she was an early model of Britney Spears.

She grins when I mention the Spice Girls. "I personally felt we had better moves. Our moves were a lot more complicated than I think you'd find in a Spice Girls video." She looks at me for confirmation and bursts out laughing. Ten years on, she is still as fresh-faced, with a huge, life-affirming smile.

I tell her she looks well. "Thanks," she says. When she was anorexic she couldn't stand people telling her she looked well; she thought it was code for fat. Not any more. "I feel fine." A couple of mighty sausages arrive for her, as if to prove the point. "D'you want one? Have one. I only want one."

She thought of calling the book Dog Years, she says. "They wouldn't let me - funny, that. It's strange, so much has happened since I left home." She hadn't considered writing a book till she heard that someone she had never met was going to write an unauthorised biography. She reckoned she might as well tell her story in her words, and get paid along the way.

Piper was born in Swindon, the oldest of four children. Her father, Paul, was a builder with a Victorian work ethic, her mother, Mandy, a sing-as-you-clean housewife. She found childhood so dull, she says. "I always wanted to be a grown-up. I was so bored with being a kid, and being at home, and having to go to school. I just wanted to work, and I found it quite suffocating. When I was younger I was so ambitious. I'm still ambitious now, but I don't have the hunger I did, say, when I was 10 years old. I was certainly more fearless then, and more selfish, and just wanted to do exceptionally well."

She was a curious confection: an eerily mature chart sensation at 15. Nothing seemed to cohere in Billie's life. In public, she was the ingénue, feigning know-how, while the real Billie was far more streetwise than she let on. When Radio 1 phoned to tell her that her first single had entered the charts at number one, she was out with her mates smoking cannabis from a "bucket bong". She was barely 14 when she lost her virginity.

Piper attended the Sylvia Young Theatre School for aspiring stars. But she got sidetracked by popstardom. When she was approached by Virgin, the last thing they wanted to know was whether she could sing. She'd been spotted on TV, advertising Smash Hits magazine, blowing mighty bubbles with her gum and shouting "Pop". She was perfect. Her singing was irrelevant. After all, she would mime when she appeared "live", and they could beef things up in the studio. This philosophy helped give her a break, but ultimately it did little for her self-belief.

She'd never thought that she had a brilliant voice, but she did think she could hold a tune. Until then. The longer she didn't sing live, the more paranoid she became. "You mime for so long that the idea of singing live starts to scare you half to death. It was in everybody's interest to mime in those days - we didn't have to have hours of rehearsals at the television studio, they didn't have to get a band in, and because we didn't sing live we could do lots of dancing, and it looked great. But miming gives you the fear of God, and you build it up in your head ...

"You have to be quite a hard person to exist in that world. You have to really want to be a pop star and make music to be in that industry, because you just say goodbye to any sense of reality. It made me feel a charlatan."

There must have been some good times? Yes, she says, and mentions the Smash Hits tour with the other bands. "It was just like being back at drama school: loads of kids, big personalities, everybody staying in a hotel and knocking on each other's rooms and sharing the mini-bar, going out into town and meeting the locals, having a laugh. Those are my fondest memories."

But Piper went on to make a fatal mistake for a young, female pop star. She had been warned not to go out with a member of a boy band because it would cause resentment among her own fan base. She thought this was ridiculous, and anyway, how would her fans find out that she was going out with Ritchie Neville from 5ive? But, of course, they did. And that killed off her first stint as pop princess. One night, touring with the other prodigies, she took her turn on stage, and found herself being booed. And the booing went on - night after night. "It was fuckin' awful. Yeah, that wasn't nice."

Did it help that she knew why they were booing her? "No, not at 16 years old. You take everything personally at that age. I was quite aware it was because I was going out with Ritch from 5ive, and he was part of the biggest boy band at the time, and he was a very desirable man, but then it did get personal. It was, like, 'you bitch' and 'your fat thighs', and so on. And I'd really cling on to that because we love to cling on to something negative, don't we? It's the good things you just forget about."

She comes to a stop and gives herself a good scratch - not for the first time. It's time to dispense with formalities. William, I say, I don't mean to be rude, but you have been doing rather a lot of scratching. She smiles, and scratches some more. "My leggings are quite itchy! Can you believe I'm wearing leggings? I like that you call me William. My best friends call me William - William of Sport." Why of Sport? "I'm not sure. I think it's a character in a play or something. I've never asked. I just like it."

Fame led to family friction. In the early days, she was too busy living out the dream to make time for her parents. When things went wrong she blamed them, when things went right she ignored them. "I took a lot of stuff out on my family, because if I had a go at execs, that was frowned upon, and the more difference there was between me and my family, the easier it got to punish them and blame them for things. But now the book has caused us to confront a few issues, which is really good. And I get really emotional now when I see my mum, like, 'I'm so sorry, I had no idea ...' "

What is she most sorry about? "Shutting them out, I suppose, and making them redundant as parents when, as a parent, I imagine you just want to be there for your kid. They didn't know anything because I never communicated with them. The only time I'd call them was when I was in a state. It was just so selfish and irresponsible, and that's the stuff I'm sorry for. I don't really regret anything I've done apart from that." In 1999, her parents sold their story to a tabloid newspaper, expressing their fear that they had lost their little girl, that she was living an unhealthy lifestyle, and saying they wished she had never been picked out for stardom. For a long time afterwards, Piper didn't speak to them.

Ever since studying at Sylvia Young's, she'd had an uneasy relationship with food. In that world, she says, it was almost weirder not to have eating issues than to have them. Things got worse. She would starve herself to ward off bitchy comments. She took laxatives by the handful, and complemented them with sleeping pills so she could try to sleep herself thin. The less she ate, the less energy she had, the more paralysed and depressed she became. She read that eating tissues was a way of eradicating the hunger pangs without piling on the calories. But they just made her choke.

How close did she come to killing herself? "Close, maybe about twice. Not just like sitting there and taking an overdose, but just like when you've got nothing left to give. Like you're already gone in a way. Mostly with the eating stuff because I'd just collapse all the time." She had gone from close on 11 stone to seven and a half stone.

She was alone in a hotel room in Chicago when she lined up a bottle of pills, preparing to overdose. Instead, she found the strength to call her parents and ask them to take her home. She had released four singles, had four huge hits, and she was desperately unhappy. That was when she retreated from the pop world for the first time.

We're at the studio, and it's time for Piper's photo shoot. She has an amazing facility for transforming herself quickly and unfussily into something special. It's interesting how her strong features make for a delicate beauty. She is talking about how dense her eyebrows are, how they used to be a monobrow. "On camera they look like the old man from Sesame Street ... Umm, I have to work a lot at facial hair. Yeah!" Where are you facially hairiest? "My eyebrows, I suppose, but the 'tache is definitely a close second?" Do you shave? "No, I don't shave, I Immac. If I'm on holiday and I let it grow, I can do this when I'm concentrating." She twiddles an imaginary tache pensively. Frida Kahlo, she says, that's who she models herself on.

I tell her I've become obsessed with ear hair. She nods, empathetically. "I think I've got five years until that starts happening."

Piper is stretching, cat-like, on a Vivienne Westwood union flag rug, as the photographer snaps away. There has always been something very British about Billie Piper - at times she could be an Enid Blyton tomboy; at times she is almost matronly. What does she associate Britishness with? "Baked beans, pubs and people with open minds. Ish. Sometimes. I'm very proud of being British. I do find people more liberal here than in most places in the world."

How would you describe yourself politically? "Oh, shut up. I'm not talking politics."

She was 18 by the time her second and last album came out - more woman than girl, freshly sexed up for an older, more male audience, and by now "Billie Piper", not just "Billie". It gave her a final number one hit, and then she disappeared again. She went on Chris Evans' radio show to promote the record, he was dazzled, flirted with her like crazy and asked her to marry him on air. She thought he was joking, and said yes. "I was, like, 'Hahahaha! OK!' Oh, we're on a flight to Vegas!" She pauses. "I'm really impulsive," she says, unnecessarily. "I just do things. I'm still like that now. Whatever feels right, and then I'll sit at home and think, oh God, that was terrible."

The marriage, though, is something she doesn't regret. "I'm really happy I did that, even though it didn't work out, because I've made a best friend for life." The whole country seemed to bear witness to their relationship. Evans, a young 34 at the time, was treated as a child snatcher because of the 16-year age gap. The day after they met on the radio show, he showed he was serious - leaving a new silver Ferrari outside her house filled with red roses.

"You know, cab drivers love that story," she says. "If I ever meet a black-cab driver, that's the chat we always have. Mmm. They just want to repeat it to me. 'You got a Ferrari, didn't you? It was full of roses, wasn't it? That Chris Evans got it for you, that ginger guy, yeah.' I don't ever think about the Ferrari moment, ever, unless someone mentions it. It made me smile, but not as much as the more intimate stuff which makes me smile all the time."

Did she drive the car? "I reversed it." And that was it? "Yep. I couldn't drive. We traded it in when we got together. We got three classic cars for it."

When they ran away to Vegas, they were running from so much - not least two successful careers. Evans was not only a top TV/radio frontman, he was a phenomenally successful businessman. Piper had just got her pop career back on course. For the next two years, they forgot about ambition, celebrity and hygiene as they holidayed. Occasionally, they were spotted: unwashed, with matted hair, pissed, content. They did daft over-the-top things, like buy Lionel Richie's multimillion-dollar Beverly Hills house, then flog it a few months later when they realised they'd rather live in a cottage.

Piper talks of Evans with such tenderness. "He saved my life," she says simply. "We found each other at a time when I was just a mess, and he was a bit of a mess, and we just made each other happy, and that's what saved my life."

Together you seemed to recover your youth? "Yeah, I suppose we did," she shouts, excitably. "We could just be children together; free-spirited. Not like juvenile, just in spirit and nature and friendship. It's like when you're young, you make friends and you're inseparable for ages. You know, it's like one of those relationships from a Judy Blume novel where you can't have enough of someone, but then you know ... as you grow older, you can, you can just let go." She stops to consider what she's just said, and giggles. "I'm sure that he'll be very happy I compared our relationship to one from a Judy Blume novel."

Was it as platonic as the relationships in a Judy Blume novel? "I think the friendship was the most important part, but there were lots of things going on in our relationship ... I really liked rubbing his hair." In her book, she describes how he educated her - buying her every film from a list of the 100 greatest movies. I heard, too, that Evans would read poetry to her when she was on the loo with tummy ache. Before I met Piper, several women said to me "What did she see in Evans?" I repeat this and, for the first time all morning, she takes exception. She looks hurt and angry.

"Why? He's amazing. And if they spent, like, a minute with him they would want him, I'm sure. If you spend five minutes with him, you just want to spend for ever with him. He's like no one else I've ever met. He has very different traits, qualities. He's just ... just ..." She searches for the word, determined to get it right. "He's just so positive all the time. I suppose that's the real difference. He turns everything into an optimistic thing. A lot of people think he is cynical and dark. I like correcting people about him. It offends me if people say, 'What was that about?', like it was just a mad moment in time that didn't mean anything. That really pisses me off."

After three years together, they separated. She's not quite sure what happened - they didn't fall out with each other, they still loved each other, there wasn't anybody else, they just grew more independent. It's as if she suddenly remembered that she was still a young woman with her life ahead of her. When she was offered the part in Doctor Who, she knew it would mean nine months apart. In the end, it was Evans who went to see her in Wales where the series was shot, and said something was wrong. They separated, with no bitterness. She recently said she wouldn't dream of claiming any of Evans' estimated £30m fortune when they divorce; that "to take a penny would be disgusting".

It's hard not to feel a little sorry for Evans, from the way she describes the end of their marriage in the book. She sees herself as an injured bird, now recovered, ready to fly the nest. And while she's done just that - and with some style - Evans seems to have been left behind, still vulnerable.

Piper is now living with Amadu Sowe, a 29-year-old lawyer. In her book she says nothing about Sowe, but in the credits, she thanks him "for the Tantric" - a joke, she says, possibly. But there's another twist to the tale. Evans lives across the road from them. How does Amadu cope with Evans as such a close neighbour? "He's fine with it. He's another amazing man."

Shortly after she and Evans split up, Piper briefly became anorexic again. But now she's confident she has conquered that. She recognises the absurdity of body dysmorphia, and is happy to mock her own aspirations towards the skeletal. "It's so mad, now, that I felt like that." She calls it a cry for help, a twisted attempt to regain control of her life.

Does she think she will marry again? "You know what, when I first broke up with Chris I thought, yes, I can definitely see myself getting married - loads more times, probably. But actually, now I can't see myself getting married in the near future." Getting married loads more times! Is there a conservative streak doing battle with her inner wildness? "Yeah, big time. I love being domesticated. Most days when I'm not working I set aside four hours to clean my flat." Four hours? Every day? "Yeah." What's that all about? "Control, OCD, a really house-proud mother, a really house-proud gran. We all make beds like they do in hospitals." She admits it's ridiculous. "I go over everybody's cleaning, even the cleaner's. I clean before the cleaner comes, and go over it when she's finished." Piper even has a trick to help Amadu clean up his act. She buys him battery-operated cleaning gadgets. They keep him amused, and by default the house ends up tidier. These days, she says, things are as good as they have been. She loves her life, her work, her cleaning - even herself.

Now she's preparing for her theatre debut early next year in the Christopher Hampton play Treats. "I'm crapping myself. David Ten-inch [her name for current Doctor Who David Tennant] told me the other day - I love him because he's so frank - that press night ..." she bursts out laughing, "they say that on press night you have as much adrenaline as you do in a car crash. That's music to my ears, David."

It is amazing how successful the born-again Billie Piper has been. She redefined the role of Doctor Who's sidekick, making her every bit as central as the Doctor himself. Why is Rose so loved? "Probably because she's like one of their mates. The girl down the road, that's her appeal, I think."

She recently appeared on the Jonathan Ross show and said that when she saw her replacement in Doctor Who she became the Green-Eyed Monster, crazed with jealousy. So why did she quit the show? "I knew that if I stayed doing Doctor Who any longer I would become more fearful about leaving and trying other things." Much of her life since she came to prominence has been spent combating the fears that came with celebrity - of rejection, of stagnation, of being exposed as a fake. And she's managed it brilliantly, even if it has meant walking away from the very things and people she loves most.

She is so different from the fearless little girl, wishing away her childhood and itching to sign that record contract, 15 years old going on 35. In a way she's come full circle. It's ironic, really - as little Billie, the popstrel in the 1990s, she would pretend that she was older, and now in Mansfield Park and The Ruby In The Smoke she's pretending she is younger; in fact just as young as when she was in her pop heyday. As for her real self, it might have taken her a good 20 years, but she's finally beginning to feel her real age and that, she says, in its own quiet way, is pretty wild.