I'd sell my soul for good comedy

Brydon now lives just down the road from his wife and three children, aged eight, six and three, in south London. "I probably see the kids as much as a normal father," he says, adding that he and his wife get on awfully well now. "To be honest," he continues, "we never got on that badly."

He does confess, however, that as co-writer of the show, he is prepared to use moments from his own life: "To a degree I'm prepared to sell my soul. There's an episode in this series where one of Keith's sons breaks his arm. That's lifted from when my own son broke his arm. Not a lovely day, that one, but even as it happened I was thinking, 'Hmm, I can use this.'"

Keith Barret's story unfolds through a series of monologues delivered to a camcorder on his dashboard. So subtle is the humour, some viewers initially believed it to be a real-life video diary. The first series, co-written, produced and directed by Brydon's former classmate from the Welsh College of Music and Drama in Cardiff, Hugo Blick, was made up of ten 10-minute shows, and netted the duo a South Bank Show Award. Brydon also picked up a British Comedy Award for his performance. The partnership worked so well they decided to do a second series.

Success was all the more sweet given that until three years ago, Brydon considered himself to be "washed up". After leaving drama school in the late 1980s, his jobs included a stint as a DJ for Radio Wales ("I was Alan Partridge"), hosting a game show and roving reports for BBC Wales, which included "sand-yachting, driving a steam train and interviewing a person who kept parrots in a small house in west Wales". Lines like these are delivered as deadpan as those in Marion and Geoff. His humour is so dry, you're never quite sure if he's joking or not. Indeed, there's an awkward moment when Brydon - who admits to having insecurities about his appearance - sits down to have his photo taken. "Um, I've got a huge boil on my neck," he says. "I'd rather you avoided that side." Assuming he's joking, I start laughing. "Er, no," says Brydon. "I'm serious."

In person, he's a wiry, compact figure and better-looking than he appears on screen. Unlike Keith Barret's clipped drone, his voice has only the faintest of Welsh accents. He's a superb mimic and charming company. He is also, every now and then, scarily like his comic creation.

Indeed, Brydon's admission of art imitating life is brave, given that Barret is, to use East End parlance, a total muppet. His talent for self-deception surpasses even that of The Office's David Brent. I ask whether he feels guilty about being part of the current vogue for depicting borderline mentally ill characters (Brent, Alan Partridge) whose lives are falling apart before our eyes. Cracking up over people cracking up, if you like.

"Yes," he nods. "That's absolutely right. The only thing in my defence is that I didn't jump on any bandwagon - Keith was a character I'd had for years, which Hugo and I simply developed. And to be honest, I don't think I've ever played anybody I've not empathised with." Brydon admits he's enjoying the plaudits and is, to some extent, "coasting" at the moment. Since the success of Marion and Geoff he's been seen in Cruise of the Gods (opposite Steve Coogan), The Way We Live Now, and Human Remains.

"I'm content with my life at the moment," says Brydon. "I have a very good relationship with my wife, even though we're separated, a good relationship with my children and a girlfriend I'm very happy with. I also have enough work. The only goal I have now is to act with Al Pacino."

Brydon's girlfriend is a researcher on The South Bank Show. They met at this year's awards. He hoots with laughter when I ask if she's putting a good word in with Melvyn Bragg to get a South Bank Show devoted to the genius of Rob Brydon.

With all this harmony and success, will he continue to be drawn to down-trodden-loser roles? "I'm sure there's a reason you end up playing the parts you do," he says. "I was a big Rocky fan. And the thing about Rocky was that he'd take a huge beating and then at the last minute rise triumphant. I found that very appealing."

And with this, he allows himself a small but well-deserved victory salute.