I am sitting waiting for my interviewee in a faceless office at Elstree Studios in an unremarkable suburb of London. The next-door studio houses the set of Who Wants To Be A Millionaire? Whoever said making TV was glamorous?
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 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.
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 outre sexual fantasies in Secret Diary Of A Call Girl.
"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.
"You have to make foul jokes and you have to tear each other apart. It's the only way to get through it."
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 No 1 with her debut single, Because We Want To, aged 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.
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." 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. For the sake of the book, he encourages her continually to try out 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 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 January 28 at 10pm