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Mark Watson may not yet be a household name, but he is still the highest achiever the Edinburgh Fringe has seen this decade. Two years ago, he was playing his first solo show to 60 people a night. This year, he’s packing out one of the Fringe’s biggest venues every night – and tearing up the room as if nothing could be more natural. Tonight he begins the latest of his 24-hour megashows. Age 27, he’s an Edinburgh institution. Oh, and he’s also written two novels, got his own Radio 4 show, and next month plays a week in the West End.

He’s ambitious, then? “Yes, yes, very ambitious,” shrugs Watson, a more measured, less Welsh proposition than his enthused onstage self. “Especially having come out of Oxbridge, there’s quite a shamefaced feeling about this; people tend to look down on ambition. Recently I’ve been a bit less shamefaced about it. But then, nearly every comedian is ambitious, really. It’s like being a tennis player – if you have to do it, you have to really do it, otherwise there will be people who sweep you aside.”

I first saw Watson at the Fringe in 2001. Playing the Grim Reaper in a Cambridge Footlights show, he had a huge comic presence. He stood out rather in the way you imagine John Cleese or Peter Cook must have done in their day. But he only joined in the final year of his English degree, after initially finding Footlights “too intimidating”.

Already in place was the Welsh accent that he performs in today. Although his mother is Welsh, he grew up in Bristol. His real voice is more RP than Rhondda Valley. It’s a curious career choice, shackling himself to a voice that is not his.

“It’s unusual, isn’t it?” he nods. “But it’s so instinctive, I genuinely don’t notice that I’m doing it now. It’s no adjustment at all, and I’ll come off stage and talk to people in the Welsh accent now.”

The Welsh Watson is garrulous, a master of observation yet disarmingly innocent. The acute observational material and the personal indignities are the real thing. The way he says them is not. “It relaxes me,” he says. “Its such an ice-breaker of an accent you get away with more.”

There’s also a third voice: the more muted Welsh tones he adopts for TV panel shows such as Mock the Week, where he can’t waste time there luxuriating in his lilt. Watson would ideally just tour – “I’d rather play to 150 people who have come to see you rather than to a million people who have just sort of ‘got it on’.” But he knows that he needs to build a name. “An awful lot of 15 to 19-year-olds watch Mock the Week,” he says. “There’s no denying that it’s been very useful to me.”

Watson has never had a proper job. In the year after university he wrote his first novel, Bullet Points. Since then he’s been on the circuit, as well as writing another book, sitcoms and sketches. Now he’s reached the point in a comic’s career, he says, where you can start turning work down.

The hero of the Fringe is even contemplating taking a year off from the festival – if he can resist the lure of adding to his Edinburgh innovations. Last year he did a show in which he wrote a novel with his audience. This year he is co-hosting a midnight quiz show with fellow ex-Footlighters Tim Key and Alex Horne. “It’s easy to moan that Edinburgh has become more and more industry-fixated, more and more a trade fair,” he says. “All that is probably true, but the only way you can get round that that is by doing ludicrously uncommercial, odd things.”

Such capers’ lack of commerciality has helped to establish his name. There were precious few other candidates for the first Panel Prize in last year’s if.comedy awards – a “spirit of the Fringe award”, or, as it was known around town, “the Mark Watson Award”. He’s dubious of the merit of such prizes. But he keeps this on display in his living room.

“Six years ago,” he says, “I was doing a Footlights show. If I had made a plan for myself back then it would more or less have looked like this.

“There are a lot of performers my age who are only just getting started now. I’ve been lucky to get a head start on those people. But the flip-side is there are a lot more comedians who have more experience of actual life than me. I’ve got to find things to talk about based on the paltry amount of life experience I’ve had.” Somehow, you think Mark Watson will rise to the challenge.

Mark Watson is at the Pleasance Courtyard (0131-556 6550). Mark Watson’s 24-Hour Jamboree To Save the Planet starts tonight at 11.30pm (www.myspace.com/crapattheenvironment). We Need Answers is at the Pleasance Dome. He plays the Apollo, W1, Sept 10-15 (box office: 020-7494 5070)

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