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Before I meet actress Sophia Myles, I’ve watched her screen debut in the new series of Spooks, which goes out on Monday. Playing an impoverished Russian prostitute, in her first scene she is bedraggled and half-starved on a Somali pirate ship.

When Myles walks up to me in Brown’s tea room — demure, blonde, very English — she is obviously nothing like her screen character. But then her role in Spooks isn’t quite what it seems either; and by the end of the episode we find out that she is not a prostitute but a spy called Beth Bailey.

With the end of the Cold War, many of us might have assumed that spies are now the stuff of fiction. But with the death of MI6 codebreaker Gareth Williams and the unmasking of red-haired Russian spy Anna Chapman, spooks are incredibly topical again.

“I’ve thought a lot about what makes the show a success,” says Myles, “and I think it’s because, in the current climate, a lot of people are constantly looking over their shoulder for danger. And the message Spooks gives out is that there is a team of people fighting to keep us all safe.”

A vicar’s daughter born in Notting Hill, Myles was first spotted by writer Julian Fellowes in a school play, age 16, and cast in a TV version of The Prince and the Pauper. Roles followed in Mansfield Park, Oliver Twist and Nicholas Nickleby. Critics quickly dubbed her the new Kate Winslet.

But actually Myles, 30, is not an English rose. “I’m half-Welsh, half-Russian. My maternal grandmother is Russian. I’ve very much a mongrel, which is good in a way because it makes me quite a blank canvas,” she acknowledges.

Along with Iain Glen and Simon Russell Beale, Myles is one of the new characters brought into Spooks after the death of lead character Ros (Hermione Norris). With her blonde good looks and voluptuous curves, she is undeniably the totty in the show but what drew Myles to her character — based on a real-life English spy Myles met through Kudos, the production company behind Spooks — is the fact that she does a grown-up job.

“Hers is an incredible story. She’s now 31 and when she was 18, she was studying English at Edinburgh University, and she got recruited by MI6, so she went through the entire fast-track selection process, and was fully qualified but at the last minute she decided she didn’t want to work for the government.

“She was still fascinated by security and intelligence so she started working in the private sector. It took her to Colombia and then most of her twenties were spent in Iraq. By the age of 26 she was a shareholder in a company worth a couple of million, with 600 ex-SAS soldiers working beneath her.”

It’s the first role Myles has played without a fictional romantic relationship. “And that has been very empowering, actually,” she says.

We haven’t seen Myles on our screens for a while. She spent two years in LA starring in US vampire TV series, Moonlight. But now she’s back based in London, in her flat in Queen’s Park. “Frantically homesick”, she missed corner shops and proper tea and being able to walk everywhere.

“Spooks was the perfect job at the perfect time,” she laughs, over a bite of egg sandwich. Myles is quite a tomboy and had it written into her Spooks contract that Beth doesn’t wear high heels during running scenes. “That was my main stipulation because I’ve done so many shows where I got badly damaged feet running nine hours a day. I said, She’s a sensible woman, she wouldn’t go into crime scenes in stilettos’. I never wear them in real life — they’re like Chinese foot binding, I’d have them abolished if I could.”

She’s an actress not a model, she insists, describing the horror of trying to squeeze into teeny runway samples for magazine shoots. In the States she ended up wearing hair extensions because her role in Moonlight destroyed her very fine hair. “In America they want you to look like you’ve just stepped out of a beauty salon in every frame. Everyone has hair extensions. You know how everyone raved about Jennifer Aniston’s hair in Friends? It’s all fake. No one has their real hair, so it’s such a ridiculous illusion.”

And she has a feminist attitude to beauty. “I’m really a strong advocate of ageing because the messages that the media and advertising give to women infuriate me: ie that it’s a bad thing to get old. So here’s a cream that will help you stay young because young means success or love’, which is just bollocks, really.”

Age became a big issue for Myles when, at the age of 21, she met Charles Dance — 34 years her senior — on the set of Oliver Twist. His marriage ended and they started a relationship. When Dance moved on two years later, Myles was reported to be devastated.

Then in 2006 she played Madame de Pompadour in an episode of Doctor Who (“The Girl in the Fireplace”) and ended up dating David Tennant for two years. She declared herself “crazy in love”, but the relationship broke up because she was based in LA. Did she find the press intrusion distressing? “Do you know what, it all feels like such a long time ago,” she says, in genuine surprise. “It’s not relevant right now.”

Yesterday Myles announced she was engaged to Brent Thomas Funston; a 36 year old entrepreneur from Texas.

“I’m in a very good place personally. I think in my twenties I wanted to be gallivanting around the globe and meeting everyone possible, but now I’m finding the desire to put roots down more permanently,” she added.

She says she realises that when she was younger she was looking for someone else to come along and complete her. Eventually she realised she had to do it for herself. When Moonlight ended after the writers’ strike, she went backpacking across South-East Asia on her own for a year.

“I was always a little bit resentful that I never had a gap year,” she laughs. “It was the only part of university I felt I missed. And it was really interesting to get a perspective on my life without it being about career or relationships or any of the external material things. Taking it back to basics grounds you.”

After all, she’s been a “professional nomad” since she was 16, and acting prompted her to turn down a place to study philosophy at Cambridge. Her Hollywood break came when she was cast as the wife of Johnny Depp in From Hell (“I think God spent a little extra time on him,” she recalls with a grin).

In 2004 she won the role of Lady Penelope in the film version of Thunderbirds (she had lessons in walking in heels and was dressed by Chanel). Vanity Fair dubbed her the next big thing. Ridley Scott gave her the lead in Tristan and Isolde opposite James Franco.

But for critics, her breakthrough role was in Scottish film Hallam Foe, opposite Jamie Bell, which won her Best Actress at the 2007 British Independent Film Awards. Cast completely against type as a feisty, promiscuous hotel administrator (with a strong Edinburgh accent), it was the antithesis of her usual “corset” roles. “I took Jamie Bell’s screen virginity,” she laughs triumphantly. “I’m very proud of that film but I always say to friends, Just don’t watch it with your parents, please!’”

For years she played virgins and victims. But now she is relishing playing more adult roles. “I was often the wronged young maiden and now I’m the kickass bitch.”

Beth isn’t a bitch, she says quickly, but she’s bloody tough. We’re all fascinated by the actor chemistry on Spooks, where so many stars end up having relationships (Keeley Hawes and Matthew Macfadyen, Miranda Raison and Raza Jaffrey). Myles lights up talking about her co-stars. She reveals that Peter Firth (Harry) is a champion tease. “He’s so naughty. His on-set hobby is making me corpse. On Spooks you have to stand there looking as serious and cool as possible and suddenly out of the corner of your eye, you catch Firth flicking a camp wrist at you,” she dissolves.

Meanwhile Richard Armitage is coaching her in how to answer journalists’ dumb questions about her beauty routine. “We joked I ought to say I frequently piss in my own bathwater, and that seems to be the secret for great skin.”

Myles would love to do some theatre. “I’ve just worked again with Ed Hall who directed the last two episodes of Spooks, and he now runs the Hampstead Theatre.”

She describes acting as almost a meditative state. “You’re constantly in the present, it’s very freeing — I live for that moment. I couldn’t care less about walking down the red carpet in a pair of heels and a posh frock. I’d rather be in my pyjamas at home.”

Series 9 of Spooks starts on BBC1 at 9pm on Monday.