'HELL-O, it's Roger Allam here." Even without the introduction, the voice on the telephone, rich and yummy as millionaire's shortbread, would be immediately recognisable as belonging to one of Britain's best-kept acting secrets.

As even a fan website acknowledges, Allam "is one of those actors who seems to be in a lot of things but no-one really seems to know who he is".

For starters, he has played a Tory MP in TV's The Thick of It, an adviser to Margaret Thatcher in The Iron Lady, a writer in Tamara Drewe and a private secretary in The Queen with Helen Mirren. And let us only touch on the theatre career of this triple Olivier winner: Les Miserables, Privates on Parade, Afterlife, Money, Blackbird in Edinburgh and Falstaff at the Globe. Then there's the radio work.

Does he enjoy being such a well-known unknown? "Oh yes," says Allam, laughing. His latest non-starring vehicle is Ken Loach's The Angels' Share, which won the Jury Prize at Cannes last Sunday. In this bittersweet comedy set in Scotland about whisky, redemption and the power of friendship, Allam plays a whisky connoisseur.

The preparation was gruelling, apparently. "I had a serious and rather drunken research session with the great Charles MacLean, who took me through the history of whisky and malts. I can't remember a thing about it now. In fact I don't think I remembered a thing about it the following morning. Very, very entertaining."

This is Allam's second time working with Loach after The Wind that Shakes the Barley. He can chart his own life through his admiration of the British director's work, from seeing Cathy Come Home as a child, then Kes, followed by Days of Hope at university. "All those things had a huge impact on me."

So when Loach asked him to do The Angels' Share he didn't hesitate. Loach, he says, still manages to make proper, good-looking films on very low budgets. All the money goes on the screen, and it comes from many sources.

"They get little pieces of money from all over Europe and different places so no-one's got a huge amount of control on it other than the people making it, which is to be hugely applauded."

As in many a Loach film, The Angels' Share features a cast made up of unknowns besides more established actors such as Allam and John Henshaw. Among those making their debut is Paul Brannigan, who plays Robbie, the one time bad lad who desperately wants to be good.

Allam was impressed. "He's very there and very present and he's got this absolutely beautiful face, perfect for the role." The Loachian mix of experienced and not so experienced works, he says. "It helps achieve the kind of freshness and awkwardness of life."

At the premiere of the film in Cannes, Loach complained that he had to cut some of the swearing in the film to get a 15 certificate rather than an 18. Allam shares the director's dismay.

"That's the way people talk and it's got its own beauty. I've just been filming The Thick of It and that has very elaborate and imaginative swearing of an order that is far, far beyond The Angels' Share." His advice: "Just calm down about it really."

Ah, The Thick of It, Armando Iannucci's television comedy about everyday Westminster folk. As Coalition Government Minister Peter Mannion, Allam is the epitome of an old school Tory trying to survive in these spin-doctoring, blue sky-thinking days. How will Peter fare in the new series?

"All I'd be allowed to tell you is that it's not going terribly well."

Mannion gets to do some swearing, but it's nothing compared to Peter Capaldi's Malcolm Tucker, the dark lord of Labour spin. As the enemy, Mannion doesn't have scenes with Tucker, but Allam knows men and women who do, and who have lived, just about, to tell the tale.

"People do say it can get pretty scary actually," he chuckles. "The whites of his eyes when he's in one of his furies literally go red. He gets the devil eyes."

Having worked at the Citizens Theatre in Glasgow and attended many an Edinburgh Festival, Allam is in tune with the Scots accent, whether Malcolm Tucker's or the ones in The Angels' Share. The Edinburgh production that stands out for him is Blackbird, written by Scotland's David Harrower and helmed by the acclaimed German director Peter Stein. The play premiered in the capital in 2005 before transferring to the West End.

He remembers Blackbird fondly because of the rehearsals in Umbria, Harrower's writing and the chance to work with Stein. Then there was the location. "What I loved about Edinburgh was being able to walk to work through a beautiful place. As I always am when I visit I was incredibly struck by how beautiful it is, the perfect size for a city, big enough to be sophisticated and have lots of interesting things going on but small enough to be practical and get around fairly easily. Also the fact that you can see hills and the sea from the centre of the city."

Among his recent theatre work, playing Falstaff at the Globe remains memorable, again because of the location.

"The perfect part to play at that address." The great thing about the Globe, he says, is that half the audience, 700 if the place is packed, only pay £5 to get in. "That gives a totally different feel to playing to people who have paid £50 to sit in the stalls."

He's not a fan of playing to corporate jollies then? "Corporate jollies are generally speaking the kiss of death to an audience. People are not there to see the thing, they're there for other reasons. It can be good, but generally speaking I would say not."

Though Allam has the smooth tones of someone who is to the manor born, his father was a vicar who came from a poor part of London. His grandfather's generation were builders and milkmen, his dad's generation were vicars and customs officers, and his are teachers, social workers, and in Allam, an actor.

In September it's all change again for Allam when he plays a detective inspector in Endeavour, a television drama about the early days of Inspector Morse.

"I'm the guy who adds experience and heft to young Morse's sensitivity and intellect."

Now 58, the lifelong Londoner and father of two has built a successful career without having to follow other British actors and go to LA. He probably never will now, he says.

"It never occurred to me when I was the right age and it would be hugely impractical now. I've got fairly young children, I don't think they'd like to be uprooted over there and I wouldn't want to be apart from them."

Then there's another rub – it can take up to 10 months to film American television series, and the commitment can stretch to years.

"It's true you can end up as rich as Croesus but the thing I enjoy about acting is doing lots of different kinds of things."

His admirers wouldn't have it any other way.

The Angels' Share, previews today, on general release tomorrow.