It is the funniest thing on radio - and probably on stage.

If you are a Radio 4 listener, then I'm Sorry I Haven't a Clue will need no introduction. Could anything be more straightforward or harmonious than singing one song to the tune of another (Kung Fu Fighting to the tune of Scarborough Fair, for example)? More confusing than "Uxbridge English Dictionary" (new definitions of words designed to baffle foreigners)? Or more intriguing than the twist and turns of that classic, "Mornington Crescent", a game famously inspired by the London Underground.

If you are not a Radio 4 listener, then this is making no sense at all. I'm Sorry I Haven't a Clue is a half-hour comedy programme, billed as "the antidote to panel games".

For many years it was hosted by the jazz trumpeter Humphrey Lyttleton. Deadpan comedian Jack Dee is now nominally in charge. And while lesser comedies - Just a Minute, for example - place undue emphasis on points, ISIHAC is more concerned with clever smut, double entendre and innuendo.

So how did all this work as a piece of theatre? Well, it was pretty much like four half-hour shows bolted together, with an interval in the middle. And instead of staring at the radio we watched some balding blokes doing it live on stage.

Or put another way, it was very, very funny. Old hands Barry Cryer, Graeme Garden, Tim Brooke-Taylor and the tuneless Jeremy Hardy were exactly what regular listeners would expect, playing many of the favourite games, such as "Censored Songs", in which the innocence of a Julie Andrews classic takes on a whole new meaning as certain words get bleeped out, or "Just a Minim", a musical parody of the aforementioned show, requiring contestants to sing without repetition, hesitation or deviation. Or repetition.

And of course "Swanee Kazoo", that melodious duet of swanee whistle and kazoo that has been an iconic feature of of ISIHAC since 1972.

At the risk of blaspheming, I was going to point out that the show, brilliant though it was, suffered from being a tad too scripted, overly-rehearsed, and lacking in spontaneity. I was tempted to express these thoughts because they are true. Even the best actors find it hard to pretend they do not know the answer when they heard the question hours earlier at the matinee. And it is written on the paper in front of them. But on account of "Swanee Kazoo", I hereby remove all such criticism.

I laughed so much I could barely hold my instrument, let alone get a sound from it. This was the biggest and best departure from the broadcast version - audience participation. Every member of the audience was provided with their own kazoo and asked to play a tune that the bemused panel had to guess.

Credit to Jon Naismith, producer of ISIHAC for two decades, who provided basic kazoo instructions, and to pianist Colin Sell, butt of so many jokes. Unspoken criticisms aside, this was pure delight.