A one-girl Spice Girls

Billie was barely in her teens when she enrolled at the Sylvia Young Theatre School – where the likes of Denise Van Outen and Samantha Janus were groomed for stardom. The connections that came with the school led to Billie bagging a bit part in the Madonna movie Evita, but the real turning point came when she auditioned to appear in a TV ad for the pop music magazine Smash Hits.

The 14-year-old Billie immediately overshadowed the other girls up for the part, bowling over the casting director with her sheer confidence. And then came the fairy tale moment: a top record producer saw her in the ad and contacted her, asking if she'd be interested in becoming a "one-girl version of the Spice Girls"? Billie didn't have to think about that for long, and at the age of 15 she became the youngest female artist ever to debut at number one with her single Because We Want To.

The ginger saviour

While her first handful of singles all sold by the bucketload, Billie was soon to discover just how fickle the finger of fame can be. By the end of the year 2000 her releases were debuting at lower and lower positions in the chart, and it seemed as though her pop career was on the wane.

But then came the fateful day she appeared as a guest on the Chris Evans chat show TFI Friday. Despite the 16-year age gap between them, the pair fell into a relationship almost immediately (Evans was so besotted, he bought her a Ferrari crammed with roses – despite her not actually being able to drive).

The tabloids sneered at first, but the genuine warmth between Chris and Billie was clear. She called him the "saviour" who showed her the way out of the pop world and gave her the space to reassess her career. And so the stage was set for Billie Part Two.

Billie 2.0

During her break from the limelight Billie briefly moved to Los Angeles with Chris Evans. Mingling with Hollywood stars aroused her interest in acting, and – encouraged by Chris – she began to take lessons. By the time she returned to the UK in 2003, she was determined to start afresh as an actor, knowing full well that casting agents would take some convincing.

The first few auditions were tough – people saw her as a washed-up singer rather than a serious actress. But then came the BBC series The Canterbury Tales, in which she stole the show from seasoned pros like Dennis Waterman and James Nesbitt. Even the snootiest critics had to admit she shone, and she was soon picked to star alongside Orlando Bloom in the 2004 Britflick The Calcium Kid. It wasn't a success, but by this time Billie was respected enough as an actress that Doctor Who fans – a notoriously hard-to-please lot – were overjoyed when she was cast as the Time Lord's new assistant in 2005. Billie's reinvention (or should that be regeneration?) was complete.

Busy Billie

Since leaving Doctor Who, Billie has been so determined to stretch herself as an actress that she's carved out an unlikely niche in the world of period dramas. This is despite her once calling them "too slow or stuffy and frankly a bit boring".

Luckily for her (and for us), her period dramas have been anything but dull. In 2006 she played the gun-wielding Victorian heroine Sally Lockhart in Philip Pullman's Ruby in the Smoke, and this year she's been in a bold, gutsy adaptation of Jane Austen's Mansfield Park. There has been one downside to it all, though. Namely, the corsets.

"You are basically tied into the corset at the beginning of the day," Billie reveals, "and then you have to negotiate everything – when you pee, what you eat. I started drinking isotonic drinks because they rehydrate you without you having to go to the loo." And with two more Sally Lockhart dramas on their way, it seems Billie will be suffering for her art for some time yet.