Jon Naismith, the producer of I’m Sorry I Haven’t a Clue, is alarmed – something that doesn’t happen to him often, if his affable air of authority is to be believed. During the show’s six annual recording sessions in theatres around the country, he hovers backstage, walking on now and then to ask for a retake or to whisper a word in the ear of the show’s 86-year-old host, Humphrey Lyttelton.

But on this night in Wimbledon the crowd of 1,500 are laughing at a joke so blue it has made them hysterical. In the wings Naismith turns to his writer, Iain Pattinson, and cries out with a mixture of panic and glee: “Bit too rude, that one?”

Well, the glory of this dirty-minded but well-spoken compendium of lightning wit and dubious parlour games is that, however much it pushes its luck, it is often rude and never offensive. That’s why its three million listeners love it with a fervour that no other radio show can approach. That’s why it has sold more than 600,000 copies on CD. And that’s why its first ever live tour this autumn has already sold out many of its nine dates without a whiff of publicity.

Its running jokes include the relentless ribbing of the former TV charades expert Lionel Blair – “who can forget the time that he pulled off Twelve Angry Men in under two minutes?” – and the sexual adventures of the show’s fictitious scorer, Samantha.

But Lyttelton and panellists Tim Brooke-Taylor, 66, Barry Cryer, 72, and Graeme Garden, 64, imbue this “antidote to panel games” with such a jovial air that Naismith hasn’t received any complaints for five years. “If the same lines were delivered by 20-year-olds,” he says, “I’m sure we’d be inundated. But people just think: ‘Oh, they’re grandads . . .’ ” These grandads have been both embodying and undermining the great British panel game for 35 years. Forming two teams – Cryer and Garden versus Brooke-Taylor and whichever of the show’s younger comics is on the rota that week – they play ludicrous games ( see box) that are part-improvised, part-prepared. None of which is ever likely to impress the magnificently unwowable Lyttelton, the show’s sheath against smugness since April 1972.

And the night’s offending joke, delivered by Lyttelton with the same enthusiasm that he might have given to reading out Yellow Pages? It was a Samantha one, which had the frisky scorer going to the butchers and ending up “with Mr Dewhurst’s tongue in cider”.

Cryer, the show’s avuncular gag fountain, calls the show “blue-chip filth”. We’ll find out tonight whether Mr Dewhurst’s tongue has penetrated Radio 4’s defences. And perhaps its touring incarnation, which will be like the usual recording session except unfettered by thoughts of transmission, will be its sauciest sojourn yet.

Yet if the BBC had had its way, this national treasure might never have been allowed out to play on its own. For the idea of the tour raised the question of ownership. Garden devised the show in 1971, after deciding that his blossoming TV career, including The Goodies with Brooke-Taylor and Bill Oddie, left them no time for their radio sketch show I’m Sorry I’ll Read That Again. So he thought he’d devise a sort of mockery of panel shows to fill the same airtime but with less writing time.

He got a devising credit for the pilot, then never again for the next 35 years. It wasn’t really an issue, he says – “until, with the tour, they started getting all stupid about it”. The BBC told the teams that they could tour, but only if they didn’t use the name. “Although everybody agrees that I came up with the idea,” says Garden, “nobody is sure that the idea is a format. That it actually exists! It is like a game. It’s a Mornington Crescent of legality.”

The dispute became public earlier this year when Cryer let it slip to the press. After a brief storm of bad publicity the BBC backed down – but will take a cut from the tour.

“It was so petty,” says Cryer, “we were really quite angry. The Little Britains, the League of Gentlemens, these days people own what they do. But when we started you wouldn’t have thought of it. We were just mercenaries.”

It’s most un-Clue-like, this real-world ramraid on this gentlemen’s comedy club (“I always think it sounds like we are in leather chairs,” says Garden). It’s a show where everyone’s a target, but the arrows are made of ice-cream. Jim Broadbent, one of the celebrity fans to pay tribute on the 30th anniversary special, summed up its appeal: “If there is such a thing as the great British sense of humour,” he said, “it is here that it must surely be found, with all its absurdity, acerbity, self-mockery and filth.”

For many years, though, it was a good programme rather than a great one. “Listen to the old shows and they sound stilted,” says Cryer. “Jon Naismith transformed it.” When Naismith took the job in 1991 he was asked to compile the first of the show’s CD compilations. He listened to 150 shows, taking notes on every round. Not a huge fan of the show before, he became a zealot. He challenged the others to think up new ideas before every season. And, most importantly, he hired Iain Pattinson. “Before that,” says Cryer, “the great Humph was just being a chairman. Jon decided that this man is such a great deliverer of a line that he needs a writer.”

And yet Lyttelton is barely involved in the preparation of the show. The teams rehearse in the afternoon before a show. Lyttelton doesn’t arrive till an hour before the audience. He has a brief chat with his producer, looks at his lines for the first time, then retreats to his dressing room. And that’s that.

“The less I know the better,” says Lyttelton, taller and twinklier than his gnomic radio presence suggests. “I’m not a comedian, so my attitude has always been, ‘I’ve got a perfectly good day job as a bandleader and trumpeter; what the hell am I doing here?’ And I’ve managed to keep that up for 35 years.”

He ignores any double meanings put into his mouth. And if he does notice any of them, he certainly wouldn’t want us to know about it: “If ever I giggle at what I’m saying I immediately stop and do it again. Nothing would be worse than me knowing that they were double entendres and then performing them in a leery voice.”

The show survived the death of the regular panellist Willie Rushton in 1996, but could it do without the mighty Humph? Garden suggests that the format is “pretty fireproof” – it would change drastically without Lyttelton, but it could survive.

Brooke-Taylor agrees. “Humph is the most important component,” he says, “but the show is better than any of us.” It took him a while to come round to that idea, though. “Willie Rushton and I talked about it once and we agreed that if Humph isn’t there it’s not worth doing. And having had that discussion with Willie, you do sort of feel, it’s not Humph that’s going to be replaced, it’s me!”

They have persisted, suggests Cryer, because the show “has never been put in aspic”. Brooke-Taylor argues that the influx of younger faces since 1996 – such as Stephen Fry, Rob Brydon, Jack Dee and Ross Noble – has kept them awake, as well as bringing in younger listeners. Garden argues that the format allows them to mess up: “Part of the fun,” he says, “is hearing the sound of sweat.”

Jeremy Hardy, a stripling of 45, guests in every series. He’ll be on the tour too. But while he can bring an abrasive edge to the show – not to mention the least soothing singing voice in England – he claims the others are more likely to keep up with comic trends than he is. “People think it’s a difficult business to get started in,” he says. “I think the opposite is true: it’s very easy to get started and very hard to keep going. And so you end up with a great respect for people’s longevity.”

After the show, in an Italian restaurant that has run out of pasta, the tone is one of gregarious relief. It’s as if, for the 49th consecutive series, they have somehow got away with it again. Brooke-Taylor chooses the wine. Garden quietly manoeuvres his veal, throwing in the odd brilliant comment. Cryer, dispatched to the end of the table to smoke his menthols, tells jokes and guffaws in that familiar way. “It’s not put on,” he insists, “I’m just enjoying it all enormously. We have four children who grew up imitating Dad. The house used to echo to HAHAHAHAHA!” He guffaws again. “Oh dear.”

And while none of them exactly demurs at the suggestion that they are national treasures, they know that stardom on Radio 4 doesn’t always mean much in the culture at large. “Probably 80 per cent of the country hasn’t heard it,” says Brooke-Taylor. “And of those that do listen to it, if someone says ‘that’s pathetic’ I can see their point.”

Smug? Somehow, against all the odds, they never are. But Garden knows the worth of his creation. Even if he can’t actually prove it exists. “I think it’s what Radio 4 is all about really,” he says. “It’s not stupid but it’s silly, and the vulgarity . . . well, you’ll never hear a dirty word. I’m a big fan of censorship because it’s a great boon to the comedy writer to try to find ways round it.”

So tune it tonight, and you will find out whether Mr Dewhurst’s tongue has made its way into one deep, dark orifice that is always crying out for stimulation. The ear-hole of a grateful nation.

Games for a laugh: I’m Sorry I Haven’t a Clue’s greatest hits

Uxbridge English Dictionary Contestants offer new meanings for words, eg, Stephen Fry’s definition of countryside: “To kill Piers Morgan”.

Late Arrivals The teams are given a social function, eg, the Drunkards’ Ball, and have to devise funny names to announce. For example: “Will you welcome please Mr and Mrs Large-Whisky, and their son, Oliver Large-Whisky.” Always preceded by . . .

The Samantha Joke The erotic adventures of Humph’s fictitious scorer. For example: “Samantha tells me she has to pop out now as she does a few chores for an elderly gentleman who lives near by. She shows him how to use the washing machine and then goes out to prune his fruit trees. Later he’ll be hanging out his pyjamas as he watches her beaver away up the ladder.”

Pickup Song A contestant sings along to a record. The music is faded down and he continues unaccompanied till it’s faded up again, trying to keep exactly to the tempo of the original.

Mornington Crescent The show’s signature dish, a game without any rules – but played in high earnestness as if it were a game with all the tactical nuance of chess, backgammon and bridge combined. I’m Sorry I Haven’t a Clue is on Radio 4 tonight at 6.30pm, repeated Sunday at noon. The live tour begins on August 30 in Leeds (www.isihac.co.uk)

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