Inside what at first appears to be a 70s office block, something extraordinary is going on.

The building's actually a TV studio in Leeds, where screen stars Helen McCrory, Cillian Murphy, Sam Neill and others, are filming scenes for Peaky Blinders.

The new BBC gangster saga's based on a real-life Birmingham gang, who embarked on a reign of terror following the First World War.

While the men are called away for filming, McCrory, still beautifully coiffed and with the glamorous feather boa her character wears draped around her shoulders, is more than willing to chat about her latest project.

"I'm very well versed in criminality," she says, laughing, when asked if the gangster world is one she's familiar with.

"No, of course not," she adds. "It's a world we don't know about."

What she is familiar with, however, is the work of Steven Knight, who wrote the screenplay. He also penned Dirty Pretty Things and Eastern Promises, but this is his first drama series. It was inspired by the stories his family used to tell, of Birmingham-based gangs who would sew razor blades into their flat caps - which they used as weapons to enforce their rules on anybody who dared step out of line.

"The heroes and the archetypes... It's like a Western in many ways." remarks McCrory, 45. "It's such an interesting world and they were characters

I hadn't read before."

She plays Polly Gray, the aunt of Murphy's character Tommy Shelby and his older brother Arthur (Paul Anderson). While her nephews were away fighting in the war, she kept the family together, but since their return her status as head of the clan's ebbed away.

"She's the sister of the mother of Tommy and Arthur," explains the actress, who previously starred in

the Harry Potter films.

"The much younger sister, obviously! We don't know what's happened to the father of her own children, and we don't know what's happened to her children either - but we find out."

The beginning of the series sees Arthur take control of the gang, building up the Shelby coffers by becoming embroiled in illegal betting and various other scams. But Tommy is keen to assert his influence, and as the plot unravels, things take ever darker and bloodier turns.

Sporting extreme 'short back and sides' haircuts, the male characters cut a menacing figure. Just seeing them huddled together on set is slightly intimidating.

As well as mastering their 'look', the cast had to hone their Midlands' accents.

"I'd like to say up front that I'm supposed to be half Liverpool-Irish, half gypsy and living in Birmingham, which is why to an untrained ear, it would sound almost as if I was doing a South African accent," McCrory teases. "But I think you'll find that in 1919, it's accurate, straight on the button, as is Cillian's, at all times."

And what of Sam Neill? The Jurassic Park actor isn't playing a gang member, but instead returns to his real-life Northern Irish roots to play a policeman who's been ordered to tackle the Peaky Blinders gang.

"He's been brought in from Ulster; they were using the police for the first time to suppress the national Irish uprising," says McCrory of Neill's character. "The government were using the police as Maggie Thatcher did with the miners - as an army. So it was very controversial what the police were doing."

Aunt Polly is the latest in a long line of strong characters for the actress, who comes across as somebody who doesn't suffer fools herself.

She portrayed Cherie Blair in The Queen alongside Dame Helen Mirren, and more recently was seen as Clair Dowar, an MP who gives Judi Dench's M a hard time during an official hearing in Bond movie Skyfall.

Polly is just as bolshy and, if the series is successful, could bolster McCrory's profile - as Homeland has for her actor husband Damian Lewis.

They married in 2007, have two children, daughter Manon, six, and five-year-old son Gulliver, and share homes in London and LA.

In an era where there are supposedly fewer roles for women of a certain age, does McCrory feel fortunate to be offered so many juicy parts?

"I've always been quite lucky, but it's unusual that I'm still lucky as I get older. I'm enjoying playing these strong roles though; most of my heroes were actresses who were working in the Forties, like Bette Davis.

"And I suppose I didn't spend my 20s walking around in a bikini, so my career hasn't necessarily suffered because I didn't sexualise myself. In fact, I'm doing more sexual roles now than I ever did," she reflects.

She admits that balancing work with parenthood has made her pickier though.

"The more you work, the more your priorities change. You look at a script and you think, 'You know, it's a great script, but I've played it before and it would just be vanity to do it again', or you want to do something new and different to satisfy you.

"A script has to be really interesting because it has to persuade me to get out of the home, to be without my children for a time. For a while, I turned down a lot of stuff, purely to keep myself at home."

lPeaky Blinders starts on BBC Two tomorrow.