I am sitting waiting for my interviewee in a faceless office at Elstree Studios, which is situated between a busy road and a giant supermarket in an unremarkable suburb north of London. The next-door studio houses the set of Who Wants to Be a Millionaire? Whoever said making TV was glamorous? In order to rescue me from these drab surroundings, I think I might phone a friend.
But everything changes when, unannounced, Billie Piper suddenly walks into the office. The room is instantly the last word in glamour. The actress is dressed in full-on Belle de Jour mode for the latest series of her ITV2 drama series, Secret Diary of a Call Girl. Done up in her costume – a dazzling blue satin dress, vertiginous heels, high-class jewellery and war-paint make-up – she looks like she has already won Who Wants to Be a Millionaire?
And yet, despite being dressed to kill, the actress is very far from intimidating. Piper radiates warmth and wit – that, surely, is why at the age of just 27 she is one of our most in-demand actresses. Both men and women are drawn to that rare combination of attractiveness and accessibility. She is grounded girl next door and gorgeous sex symbol rolled into one.
It is no surprise that Piper was in sixth place in Broadcast's Hot 100 list of the most influential screen performers and was the leading woman in the countdown in 2007. The breadth of her appeal also explains why a couple of years ago Piper, who played the Time Lord's assistant Rose, was being discussed as the first female Doctor Who. The show's then executive producer, Russell T Davies, responded favourably to the idea. "I'd do it without blinking," he said. "Why not?"
As James D'Arcy, Piper's co-star in Secret Diary of a Call Girl, puts it, "Billie has a quality that so few actors possess: you instantly like her. It doesn't matter if you're 16 watching her in Doctor Who or 65 watching her in Mansfield Park – you immediately connect with her. Billie's appeal is completely natural – you can't manufacture it. Some actors are adept at generating it, but Billie just has it effortlessly."
Piper, who hails from Swindon, where her father is a builder, has a keen sense of humour. She reflects, for instance, on one of the most bizarrely amusing sequences she had to perform in the latest season of the drama, which is based on the bestselling book, The Intimate Adventures of a London Call Girl by Belle de Jour.
The actress recollects that, "we had this scene with this guy who was into having sex, not with animals, but with animals in mind. So he liked to bleat like a sheep, as I said the most crude, foul, naughty things to him! It was just really funny. But I did leave the set thinking, 'what am I doing?' I was thinking about my family as well because I can get away with this. But my dad's a builder, and he has to go to the builder's merchant tomorrow, and he's just going to be ridiculed. And yet that doesn't stop me". With a cheeky grin, she adds,"I'm a selfish woman!"
That infectious sense of humour avowedly comes in handy when acting out some of the more outré sexual fantasies in Secret Diary of a Call Girl, which begins its third series on ITV2 on Thursday 28 January. "You have to laugh when you're doing this show," Piper confirms. "The stories are so alien to us all that if you don't laugh, it becomes very intense and nerve-racking, especially for the actors playing the clients who come in just for one day. If you're very serious and saying, 'Oh, don't look at me, don't touch me there, no laughing', it doesn't work.
"You have to make foul jokes and you have to tear each other apart. It's the only way to get through it. You have to actively give people chewing gum and tell them that their breath really stinks and they need to sort it out. It is so full-on, but everyone really wills you through it and holds your hand. Sometimes, the scenes are just horrible to shoot, but it's all done with good humour."
Alongside her ability to laugh at herself, Piper manifests a winning sense of vulnerability. She reveals, for example, that in this series of Secret Diary of a Call Girl, she employed the services of that rarely used film-crew-member, the "foot double". "I hate my feet," sighs the actress, who is married to fellow actor Laurence Fox and is the mother of a one-year-old son, Winston. "They're my least favourite thing in the world. They're really crammed into my adorable shoes. They're so vulgar, and looking at them now makes me want to cry!"
Since reaching number one with her debut single, "Because We Want To", at the age of 15, Piper has led her entire adult life in the public gaze. During her three years with her first husband, Chris Evans, she was constantly at the eye of a storm of publicity,
But she maintains an admirably level-headed attitude towards the press. This is a woman who would keep her feet on the ground even if she were attached to a whole fleet of hot air balloons. "Sometimes it's a bit of a drag when journalists call you or your family," admits Piper, who attended the Sylvia Young Theatre School, also the alma mater of Leona Lewis, Amy Winehouse, Keeley Hawes, Tamzin Outhwaite and Emma Bunton. "It would be nicer just to do the job without all that. But over the past few years, I've taken a different approach. Now I get over it. I don't let it become an issue – I just ignore it. It doesn't hurt you that way.
"I don't read stuff about me anymore. If I see a picture of myself in a magazine, I just quickly turn the page. But I still read about everyone else! There's no denying that I love to flick through those celebrity magazines when I'm getting my hair done. I know it's wrong. I recognise that, but once I start reading one of those magazines, I just can't put it down!"
Piper appears to have found domestic tranquillity now with Fox and her son in rural Sussex. But she admits that it was a struggle when she first came back to work and had to leave Winston at home. "It will always be a wrench to go to work, but I think if you have a desire to work, then you have to try and do that somehow – otherwise you could become quite resentful."
All the same, the actress concedes that when you go back to work there is also a danger that, "after a while your child starts to neglect you because he feels neglected himself. He'll reject you one morning and go, 'no, I don't want a cuddle', or, 'you're unrecognisable to me, mum'. Then you feel really bad, and you have a little cry and it's all very emotional ... I'm still working it out. I haven't really fathomed the balance or the right way yet. But I've managed to do this job and see my child and my husband and also have a good time".
One other problem Piper experienced initially was how to arrive at the set of Secret Diary of a Call Girl and immediately get herself into Belle's mindset. "It was hard for me to make that shift in my mind from being someone who's being very maternal. My instincts were very different to the character's, and I found that quite tough at first."
Becoming a mother has certainly given Piper a greater sense of perspective about work. "I stop clock-watching and things like that, which I used to do on set all the time. I would be saying, 'When's lunch? When's home time?' – all that stuff. Whereas now I just get on with it and don't really feel the desire to moan or complain. Because you spend so many hours awake when you have a child, everything else in comparison is quite a breeze!"
In the latest series of Secret Diary of a Call Girl, Belle is now a published author. She becomes involved with her editor, Duncan (D'Arcy). For the sake of the book, he encourages her continually to try out more ever more exotic experiments in the bedroom.
One of these is "sploshing" – no, me neither. I'll let Piper explain what it is. "'Sploshing' is a really strange concept. I couldn't really get my heard round it, but it's where people like making love to food, like cakes and trifles and blancmange. I was very interested in how it was going to work and whether it would be ridiculous or whether it would ring true.
"In fact, we all had the best time because it just seems so crazy, but also really good fun. I'm not sure I could get turned on by that situation, but we had a real laugh doing it. I had to have beans poured in my pants, cake thrown at my face, and I had to massage Angel Delight into a client's beard and hair. These are the kind of things you wish you could do all the time – food fights are so funny. But we can't really behave like that in real life because it's frowned upon."
Piper continues that one of the main attractions of playing Belle is that the actress is allowed to do things that she would never get away with in real life. "Pretty much everything we do in this show is new to me. It's a strange subject matter, and it could come across as kind of seedy and reckless, but in fact it's just good fun. This is what happens in the world, and it's quite interesting to tell those stories. It's definitely one of the reasons why I do it because half these things people don't really know about."
The first series of Secret Diary of a Call Girl aroused some controversy – certain critics thought that it glamorised prostitution – but Piper defends it stoutly. "People really divide when it comes to our show. Some are torn between secretly enjoying it and then also going, 'but morally it's wrong'. The show also often has feminists upin arms. They often cry out and say, 'These poor women are victims, and it's so sad and so irresponsible of them to make it look like people have fun whilst they're doing this.'
"But these people do exist, and I think it's naive to ignore that these things go on in the world. Why can't you see the funny side of it? I've met these girls, and as hard as I've looked – and I have looked hard – their world isn't tinged with tragedy.
It's often just about the fact that they've needed money. They enjoy money, they enjoy the company of men, and it's all very organised. They're seemingly OK with that. I don't think people should watch it if they're going to get all shirty about it."
Piper is going on to play a woman in 1950s Leeds who has affair with her neighbour in BBC1's A Passionate Woman. She also made a fleeting appearance as Rose in David Tennant's last ever episode of Doctor Who earlier this month.
So does this presage a permanent return to the Tardis? "No," Piper replies, firmly. "I think it's a healthy time to leave. I mean, our period on Doctor Who is over now really, isn't it? It's shifted. The writers are different, the production team are different, the Doctor is different. It's time to move on.
"I can't keep re-appearing. It would just become sad. If I were still re-appearing when I was 50, people would say, 'Oh no, not you again!'"
The third series of 'Secret Diary of a Call Girl' starts on ITV2 on Thursday 28 January at 10pm