Although she gives quite the opposite impression on meeting her, there must be something innately restless about Billie Piper. Even at school in her home town of Swindon, she was desperate to get started. 'I was freakishly ambitious. I didn't want to be a child. I wanted my own flat, to work and be a grown up.' At 15, Piper was propelled, via the Sylvia Young theatre school, into a pop career, which, according to her 2006 autobiography, Growing Pains, led to her combining chart success with anorexia. Marriage to Chris Evans was followed by the most amicable divorce in showbiz history and the career-forging role of Dr Who's assistant Rose. Last year, there was more controversy when Piper took on the role of Hannah/Belle, the high-class prostitute in Secret Diary of a Call Girl.

Quite a lot for a young woman only just about to turn 26? 'Yes, I do think sometimes that I've lived about three different lives,' grins Piper when we meet to talk at a central London hotel. Six months pregnant, petite, with honey-brown hair, Piper is articulate, funny and self deprecating, as well as disarmingly friendly, pulling her clothes tighter to show me her bump. Filming the second series of Call Girl, they had to work around the pregnancy, her first child, with her husband, actor Laurence Fox. 'Towards the end, it was all the back of my head or in shadow, like something from The Phantom of the Opera,' she sighs in her soft, lilting voice. 'So embarrassing.'

Is she enjoying being pregnant? Yes, says Piper, though she is clearly nervous. (Towards the end of our conversation, we get stuck into the nitty gritty of epidurals, natural childbirth and Caesareans, but I wish we hadn't - the memory of Billie's huge, apprehensive eyes haunts me for days.)

'Some women are all about this glowing, blooming thing, aren't they?' she says. 'I haven't really tapped into that. I've basically replaced all my vices for a new one - control. I've had about 10 scans and I bought one of those baby heart monitor things. Such a bad idea - its like listening to a huge shell on the beach,' Piper smiles. 'But I do love it. The whole thing is just so sweet, tender and adorable.'

While the first series of Call Girl concentrated almost exclusively on Belle, as she embarked on everything from lesbian romps to saddling up a client, the new one delves deep into Hannah's emotional life. 'There are still sexual encounters, but now there's one an episode instead of three.' Piper grins. 'There's going to be a lot of pissed-off men.'

Was she taken aback by the furore surrounding the first series -it was accused of glamorising prostitution? 'I thought there would be a fuss but I didn't think people would be so up in arms,' she says. Piper was aware of the grim realities of prostitution. 'The drug-fuelled sex trafficking side of things that you read about in the media. But I met girls who have a very different story, especially the woman who wrote the book and the blog. They were adamant they didn't consider themselves victims.' What about the charge that it might inspire someone to become a prostitute? 'Did we? I think people have common sense and can tell what's real, what's right or what's wrong and work it out.'

In fairness, Call Girl seems so glossy, so knowingly camp that it's difficult to find it any more shocking or immoral than, say, Mistresses. Nor did it shy from the fact that Belle's true amour is cash, not sex. Piper nods. 'When I met Belle, she made it clear that first and foremost it's about the cash.' In fact, Piper suspects that money was behind the reaction to Call Girl. 'The only difference between this show and Sex and the City seemed to be the money changing hands. There is so much sexual liberation, but the minute a woman lies on her back and takes cash for it, that changes everything.'

On a different tack, surely it felt odd, playing a prostitute while pregnant? Piper confirms that she had a body double for the second series. 'My instincts had changed,' she says. 'Certain things repulsed me - just other people's bodily smells. Not that we were sniffing each other's groins or anything. It was just being that close to an actor who wasn't my husband, having people touch me. Suddenly I felt quite...' Sensitive? 'Yes.'

Her hands flutter unthinkingly near her bump. 'And more emotional. My senses were just a lot more alive, so acute and strong.'

Saying that, Piper felt bemused by the reaction to her sex scenes in the first series. 'I'm playing a prostitute. I have sex. If that's going to be a problem, I should have thought about it before filming. You can't turn up and be prudish on the day.'

It's probable that Piper's huge success in Dr Who played a large part in the 'shock horror'. People don't want to see Rose playing a whore (Or they do - which is a whole other issue.) Did Piper take on Belle to avoid being typecast as Rose? 'It's more about my always wanting to do something quite far away from the last job because I get bored so easily,' says Piper, suddenly earnest. 'That's one of the best things about acting - you know, when you get to learn about all sorts of different people.'

Born in 1982, Piper, the oldest of four siblings, was only 15 when she was handpicked for pop stardom, going to number one with 'Because We Want to'. Piper is clear eyed about her vocal talents. 'I sometimes think: would I have gone on X Factor? And no, I wouldn't - I wouldn't have been good enough. These people work so hard and they sing live. I mimed throughout my entire pop career.'

While she was a pop star, Piper became increasingly estranged from her parents. 'I had very bad phone etiquette,' she says. 'Mostly it was because I wanted to punish them. I wanted them to just be my parents and talk about things that weren't related to what people were saying about me. Now I feel awful about some of the things I said. 'Has being pregnant made her think about her relationship with her parents? 'More and more. It's funny, isn't it?'

What about the anorexia and poor body image? 'I think it was because I saw so many grown-ups living destructive lifestyles, maybe skipping breakfast, drinking copious amounts of coffee and Diet Coke, smoking fags and being slim and desirable and successful.' Does she worry it will return? Piper shakes her head. 'I know that what I go through now is no worse than what my best friends go through who have a relatively healthy relationship with food. Everyone's fucked these days, aren't they? Men too. I mean, men who talk about food endlessly - it's so depressing.'

Feeling lonely, unfulfilled, and self-destructive, at 18, Piper met Evans. The next day he sent round a silver Ferrari full of roses. Six months later they were married. A huge fuss was made of their 16-year age difference and the paparazzi photographs of Billie in her pyjamas pushing trolleys full of booze out of supermarkets. You had to wonder - was she depressed? Was she in hiding?

'People seem to consider that I had a serious meltdown,' says Piper. 'That I married an older man and got pissed in my pyjamas for two years. But these were important formative years for me. Just to relax and care about things that weren't related to what I was doing before.'

Since their highly amicable divorce (Piper refused to take a penny in alimony), they have remained close friends, Evans eventually going on to marry golfing journalist Natasha Shishmanian, with whom he is expecting a baby. Does Piper look back on her time with Evans now as a healing period? 'I do, but not in a creepy way. I don't consider my years with Chris as some kind of rehabilitation.'

That's offensive to him? 'Massively offensive. And really offensive to the relationship we had and continued to have. Even though he did help me, it's not as if we were spending all our time talking about our problems and then going out and getting wrecked.' She bursts out laughing. 'We were just going out and getting wrecked.'

Once Piper's acting took off, it seemed there was no stopping her, with appearances in productions as diverse as The Canterbury Tales and Ruby in the Smoke. She was outstanding as Fanny Price in Mansfield Park. And, of course, there is her massive, award-winning success in the revitalised Dr Who. While Piper is quick to acknowledge that Rose 'put her on the map', she seems to find her Dr Who fame equal parts unnerving and amusing.

'It can become the topic of every conversation. You'll be at a wedding and somebody will say, "Do you remember that episode?" And these are grown men! Sweet as it is, I do find it hard work, though very funny.'

More recently, Piper went to Toys R Us to look at nursery equipment. 'Suddenly, all these kids are chuckling and following you around. And you turn a corner and there are dolls of you everywhere and massive pictures. It's quite frightening.' Piper smiles wryly. 'I think it must be hard being David [Tennant]. I get a certain level of attention but - I've seen it in action - he can't move for attention.'

Where fame is concerned, Piper keeps a low profile, especially since becoming pregnant, preferring to stay at home in West Sussex with Fox (he starred in Elizabeth: The Golden Age, Gosford Park and TV's Lewis). 'I'm not trying to congratulate myself on not going to premieres, but if you don't want to court that shit every day of your life, you can avoid it.' It helps, she says, that Fox, whom she calls Lol, is 'a low-key, chilled guy. He likes doing the work, but he's not into that stuff'.

Endearingly, Piper admits to a gossip mag habit of her own. 'I know I shouldn't!' Would she mind not being famous? 'I would find it quite strange at first, if I'm honest,' she says. 'It's all I've known for the past 10 or 11 years.'

Another key role for Piper was Christopher Hampton's Treats, not only her West End debut, but also where she met and fell in love with Fox. Piper admits she found the play 'really hard'. 'I felt slightly out of my league, which is why I wanted to do it,' she says 'But the nature of the role was tough. Breaking down in tears every night, being the victim of this man's verbal abuse. And just being there every night, everybody knowing where you are.' Piper shakes her head. 'I was quite honest about the fact that I was finding it a hard experience when I was doing press. I think that fuelled those rumours that I was having some kind of breakdown, which wasn't true.

'I was also falling madly in love with this guy and that makes you quite weak and vulnerable. I wanted to be good in it myself, but also for him.'

Piper smiles shyly, pushing back a strand of hair. 'So every time I went on, there were all these things to consider and I thought: why am I doing this? It's terrifying!'

Nevertheless, she says she'd try a stage play again. 'At first, I thought I never could, but now I really would.' Her lips twitch. 'Though not for a long run.'

What about the intimidating-sounding 'Fox acting dynasty' (Laurence is the son of James, nephew of Edward, cousin to Emilia and all the rest of it)? 'Acting dynasty?' shrieks Piper with delight. 'I never know what that means. I go around to their house and they're just really loving. I don't find that remotely intimidating. If anything, I find it refreshing. I really want to be part of that.'

She's quite the homebody isn't she? 'I really am. I always have been.' Does marriage stabilise her? 'I just believe in marriage. I believe in being with someone for ever. Even though my marriage to Chris didn't work, I still don't consider it a failed marriage - it just came to its natural end. It didn't make me feel like I never wanted to get married again. I like sharing things with somebody.'

An unfair question - why didn't she have children with Chris Evans? 'I just never thought about it,' says Piper. 'I was young and I suppose we'd broken up by the time I'd have started thinking about it. It was just never an issue.' With Fox, it was different. 'When we moved in together, I did start thinking about it, on my own. You know -"I'd love to have your baby because I just think you'd be brilliant. And I would really love to share that with you." But I didn't tell him.'

Piper seems desperately in love. Does it help that they're both actors, in that they understand each other's jobs? 'I don't think anyone understands anyone's job,' laughs Piper. 'We all have the same hang-ups. I get jealous if I know Laurence is kissing someone else.' That seems a bit unfair - he has to watch you in Secret Diary of a Call Girl. 'Yes, but he's very grown up and I'm quite childish. But this actor thing - I don't think it benefits a relationship in that way. I still call up and go [she adopts a whine], "But you said you would be back at seven." Even though I do exactly the same thing for a living, I still act like I simply can't understand how he can do this to me.'

As the interview winds to a close, I realise that there's been none of the usual talk of future plans. And that's because Piper has no plans bar imminent motherhood. 'Lol is working a lot. I just imagine being with the baby on my own when he's not around. That's how I picture it.' And that's a good picture? 'Yes I like it.'

Certainly, Piper exhibits none of the usual actor-jumpiness about the next job and says she spends her days pottering about, catching up on TV boxed sets and movies (she adores the Judd Apatow ensemble players in Superbad and Knocked Up: 'They all look so nice!').

'I don't know how I'm going to feel when the baby comes, so I don't want to commit to anything,' says Piper, fairly reasonably. Indeed, any creative restlessness in her make-up seems dormant for now. 'You know, I just feel really content at the moment,' says Piper. 'I've done quite a lot of stuff and I don't have to feel desperate to do anything that radical or wild. I've set myself all these challenges and worked through them. I just feel I'm ready to be a mum now.'

· Secret Diary of a Call Girl starts on Thursday September 11, on ITV2

Source: The Observer September 2008