It sometimes feels as if Paul Merton has been around for ever. But in a good way. This interview, which took place in September 2011 was my fourth encounter with the genial absurdist. The first time I interviewed him in the early 1990s was in his agent's office in Regent Street. While other "alternative comedians" had gone with cutting edge new wave management companies, Merton was handled by a company with a lengthy showbiz pedigree. One of the agents was Peter Pritchard, who often crops up on nostalgia progs recalling how Mario Lanza punched him backstage at the Palladium. Merton's Goonish sensibility kind of fitted in perfectly and yet didn't. Two decades on he still kind of fits in perfectly and yet doesn't. BD.

If nostalgia had been an Olympic event this summer Paul Merton would have been a shoo-in for gold. As he sits opposite me in a Soho club I pull out my smartphone to record our conversation. He looks at it in the way a caveman might look at a car and slips into a reverie about the changing world. "I don't have a computer or mobile phone. If I was a 24-hour emergency plumber I might need a phone, but not for appearing on panel shows." He chuckles as he thinks back to the days when people took their holiday snaps to the chemist to get them developed: “You waited a week and when you got them they had a big thumbprint on them. It does makes you wonder where we will be in 20 years time.”

In two decades time a 75-year-old Merton will quite possibly still be spinning out surreal riffs on Just a Minute and Have I Got News For You. He has been appearing on the former since 1989 and the latter since 1990 and clearly likes a bit of stability. He has been pitching up in the West End to improvise with the Comedy Store Players for over a quarter of a century. Merton doesn't have children, but maybe the likes of Nicholas Parsons, Ian Hislop and his impro crew are, in a bizarre way, a surrogate family. "I'm a team player," he suggests. "And maybe that has helped my longevity. If you are not the solo person there is less chance of people getting sick of you."

Merton's nostalgic streak also embraces his BBC documentaries on silent cinema. He has always been something of a man out of time. When he was growing up in south London and his mates had posters of Bowie and Bolan on their wall he gazed up at a picture of hangdog clown Buster Keaton. And when he started in stand-up in the 1980s one of his first breakthrough jokes was about World War 2. “During the war, people used to say that you needn’t worry about the bombs. They would only hit you if they had your name written on them. Which was bad news for my neighbours, Mr and Mrs Doodlebug.”

He dusted off the joke again this year when he did his first UK tour in over a decade. It was an odd show which was billed as his return to stand-up, but took in sketches, impro and dream sequences featuring flying beds and toy rabbits. It got, shall we say, mixed reviews, but it did improve as it went along. "The only way you can tell if a show is working is to perform it. We had one prop that cost £800 which I thought was funny, but didn't get a laugh. So that was a waste of £800."

Over the years Merton has seen it all. During one show at the Palladium in the early nineties someone threw a pair of soiled underpants onstage. When he appeared with his first wife Caroline Quentin in Arthur Smith's play Live Bed Show there were disgruntled punters who were maybe mislead by the saucy title. One of the reasons he loves live comedy is that the unexpected can still happen. "The other night there was a bloke in the front row dressed as a giant turtle. I was a bit worried he was going to be a problem, but he turned out to be a big fan."

Maybe another reason why Merton loves old comedy is that he is an unashamed comedy obsessive but tries to steer clear of contemporary comedy for professional reasons. He cites Ross Noble and spoof character Count Arthur Strong as favourites, but doesn't want something to lodge in a dusty old file in his memory which he later thinks he has thought of. "I once did a sketch that The Goodies had done involving a horse landing on a man's head and when I realised I was disappointed that I'd subconsciously nicked that." There is also another reason why he doesn't watch much modern comedy: "You can drive yourself nuts thinking 'why is that person doing better than me?'"

And nuts is a state of mind Merton doesn't want to be in. He talked in his recent stage show about his spell in the Maudsley Psychiatric Hospital two decades ago which was prompted by a reaction to anti-malaria pills when he thought people were talking about him and he was being followed by the Freemasons. In the hospital things were still unsettling. "If you are in a normal hospital with a broken leg in a ward with people with broken legs you don't break your other leg, but in a psychiatric hospital somebody comes in believing there's people hiding in the chimney and within 24 hours others think there are people hiding in the chimney."

Luckily when Merton came off his anti-malaria drugs he felt better and after six weeks left the Maudsley and has never had an incident like that again. In fact he dismisses the idea that comedians are, by nature, an unstable bunch. "I think it's a myth. When your career is going well you've got nothing to worry about. I think Tony Hancock would have been suicidal if he had been a plumber. He'd be wondering all time time 'where does the water go?'"

Without comedy though Merton might not be suicidal, but he would certainly be more miserable. He quit his deskbound 9 - 5 job at the Tooting employment office to pursue his comedy dream and after an early West End gig that went so well he walked all the way back to his Streatham bedsit with a smile on his face he has never looked back. "I'd go in to work and try desperately not to watch the clock all day but I couldn't help it." Working in that office is definitely not something Paul Merton is nostalgic about.