TO millions of fans, Gillian Anderson will always be alien investigator Dana Scully. But these days the former X-Files star is a London school-run mum - and she's joining the ranks of the terrestrial TV detectives

Let's put the truth out there: Gillian Anderson has never looked better. Dressed all in black, in tight jeans and a polo-neck jumper, she wears her hair scraped back in a ponytail that shows off flawless skin and those famous cheekbones. You'd never believe she was 44.

She's relaxed, too, and instantly chatty, with no hint of the "spiky and serious" person she's claimed to be in the past. When she speaks - in the English accent she adopts on these shores - she's considered and thoughtful, but very aware of the clock and the minutes ticking away before she must leave to pick up her two youngest children from school.

Though Gillian Anderson will forever be associated with The X-Files, the paranormal drama series that ran from 1993 to 2002 and spawned two movie sequels, the role we're discussing over a coffee is DSI Stella Gibson in the new BBC2 drama The Fall. And Gillian reckons her character, who's called in to help investigate a string of murders in Belfast, is one of her favourite roles to date. From the lips of agent Dana Scully, that's a big statement.

"Stella instantly jumped out at me," explains the actress, who lives in London with her sons Oscar, six, and Felix, four. "Often a character can do that and the rest of the script doesn't measure up, but Allan Cubitt's scripts are so compelling.

"I love how undefined Stella is. Even by the end of these five episodes, you don't really know who she is or what her story is. She gives nothing away. She's very I get to be slapstick be independent but she's a woman in a man's world.

In that situation you go one of two ways - either you get what you want by using your femininity, or you learn to be tough and command the respect you deserve based on your expertise. Stella is more the latter."

Gillian is well known for playing strong characters - her forays into TV Dickens comprised a terrifying Miss Havisham in Great Expectations and a fearsome lady Dedlock in Bleak House - but that's not because she goes looking for them.

"Strong is just how I get cast, so it's not necessarily by choice," she explains. "A lot of the characters I've played might be strong personalities but they have equally big weaknesses, too. I don't often get offered the kind of roles where the vulnerabilities are worn on the sleeve, so I'd love someone to send me something where I get to be ditzy. Even slapstick would be welcome."

In fact she'll consider any job as long as it doesn't mean long periods of time away from her children.

"Generally when I get offered something, the first question, even before I look at the script, is what are the dates?" she smiles. "If I really like something and want to pursue it then I get the calendar out. With The Fall, which was shot in Northern Ireland, I said I could do it if I was allowed to come back for weekends and they gave me a week off for half-term in the middle.

"I've been lucky so far in that most of the things I've chosen to do are quite short-term - I've chosen a lot of smaller roles and that kind of suits me in terms of scheduling and the kids. I do have a nanny but I've never been away from them for more than two weeks."

Till now, that is. "For the very first time I'm about to go away for two and a half weeks, for Hannibal [NBC's much-hyped new series in which Gillian plays the notorious Mr Lecter's therapist]. It's freaking me out a little bit."

So is there no job offer that could tempt Ms Anderson to tinker with her work/life balance? "If it was a big film, maybe, then I'd start to think about taking them out of school and getting a tutor," she smiles. "Yes, for Scorsese that might be doable!"

A self-confessed cinema buff, Gillian's passion undoubtedly lies in film. So how come she's here and not fielding offers in LA? "I hadn't originally intended to be in the UK full time," explains Gillian, who grew up in Chicago and London and feels an equal affiliation to both countries. "I bought a house in London when I was 35 and I always assumed it would to be a part-time place, where I'd be for a few months in the year. But then I fell in love when I met my second husband [documentary film-maker Julian Ozanne] and I fell in love with London, too. Now it feels like the right place for me to be. The kids are in school, and to pick up and move to LA in order to pursue things doesn't feel genuine."

Gillian married Ozanne in 2004 but the couple split two years later. She then got together with businessman Mark Griffiths, the father of her sons, but split from him last year. She also has an 18-year-old daughter, Piper, from her first marriage - to one of The X-Files' crew - which ended in 1997. Currently single, she reckons that motherhood is a very different experience second time around.

"Looking back to my twenties I don't think I realised the importance of that role at the time," she admits. "I can say that I did my very best and I fought hard to get as much time with my daughter as possible, even if that meant that she was on set with me all day long. I feel very good about what a priority I made it, even though I was young. But there's something quite different about being an older mum and having that life experience; it feels more precious somehow. It becomes more and more important to have real quality time with your kids. Also, the more you learn about yourself and the impact your own parents had on you, you start to make different choices."

Gillian still makes sure she schedules time for Piper, even though she's at boarding school. "She comes home at weekends and we have a lot of fun together. It's just a different kind of responsibility having a teenager, as they start to become as much a friend as a child. You have to judge boundaries around what kind of information is appropriate to be shared - it's a delicate balance with mothers and daughters."

Piper is not the only legacy of Gillian's years on The X-Files. The global fame it brought her shows no sign of abating, with new generations discovering the cult show's charms, and a surprising number of girls admitting to crushes on agent Scully.

"I still get letters and emails from people who are my age or a little bit younger saying they're writing because they used to be a fan, or they're 12 years old and just discovered The X-Files," she says. "Often women and girls talk about the strength they've gotten from Scully and how the stuff she went through and the strength she showed has helped them go through difficulties in their own life. It's amazing."

The actress has good reason to be grateful to Scully, and not just for making her famous. "When I was offered The X-Files pilot I was on my last unemployment cheque - I was literally sleeping on somebody's couch," she says.

"I was young and naive and I didn't know what I was getting myself in to. I've generally thrown myself into things without standing back and assessing the situation, or asking myself if it's really something I want to do, but if I had my time again I wouldn't choose any differently."

Gillian seems to have no fear of the roles drying up. On the day we meet she's received no less than four job offers, and there's already another series of The Fall in the pipeline. But she admits that not even her flawless features are immune from the effects of time.

"I never used to be bothered about hair and make-up and all that stuff, but I've started to pay a little bit more attention and that has to be vanity," she admits. "I care a little bit more now than I ever used to about how I look, and that has to do with ageing. But I really don't try that hard to stay in shape. I go through stages of yoga a few times a week and throw in some running.

"I've worked with trainers at various times in my life - but what that does is negligible," she adds. "I'm never going to be that person who gets up at 5am every day before the kids wake up so that I can work out for an hour. I'm just too lazy. If I thought it was actually going to make any difference to the state of my thighs I might but it doesn't. The bits I'm worried about are unchangeable and I just have to accept them."

The Fall is coming to BBC2 soon.