Mischief is afoot from the very start of this very British apocalypse: "Cast in order of height" declares the credits of The Bed Sitting Room, as smaller performers like Rita Tushingham and Dudley Moore, and taller ones like Peter Cook and Ralph Richardson turn mad, or into furniture through atomic mutation, in a London turned Neolithic through technology. An A-Bomb in Wardour Street, indeed. However, as Pink Floyd once sighed, "Hanging on in quiet desperation is the English way" and, as always, small island ritual, tradition and stiff-upper lips must yet prevail, even after World War III.

Amid such bombed-out devastation (in reality, a disused quarry in Surrey), one man powers the national grid by pedalling furiously on a bicycle dynamo; Frank Thornton's BBC man, formal and proper in exactly half a tuxedo, declaims yesterday's news through a shell of a television set (and closes with a rendition of the revised national anthem "God Save Mrs Ethel Shroake of 393a High Street Leytonstone"); and Arthur Lowe's family man rides endlessly round the Circle Line with his wife and 18-month pregnant daughter, subsisting on chocolate bars purloined from tube station vending machines.

Elsewhere, Marty Feldman's cross-dressing Nurse Arthur roams the desolate terrain, personally delivering death certificates to the living: "I thought I was alive, but here it is in black and white" exclaims Mother (Mona Washbourne). "Do I lie down or something?" Nevertheless, she too has begun the inexorable Dali-esquire transformation into a cupboard ("Get your hand out of my drawers!"), just as her Prime Minister husband will transmute into a parrot - soon to be cooked and eaten by Spike Milligan's passing postman. "Guess this'll mean a by-election," he observes, gnawing on a wing.

Although, as conversions go, Lord Fortnum of Alamein (Ralph Richardson) achieves the most startling of all: he turns into a bed sitting room. "What do I take for it?" he enquires of Michael Hordern's roving GP. "Three guineas for your rent," comes the reasoned reply. And aggrieved at having wound up in Paddington, or what's left of it, Fortnum demands the doctor place a "No coloureds" sign in his window. Hovering above them all, in a clapped-out Volkswagen tethered to a hot air balloon, Peter Cook and Dudley Moore's bowler-hatted policemen bark through megaphones at the straggling survivors to "keep moving - you're safer that way".

"How long is this sh*t going to go on for?" a United Artists executive barked at director Richard Lester during a company screening. At times, contemporary audiences might ask the same question of this near-plot less, relentlessly absurdist sketchbook, overwhelming and fatiguing in its obsessive-compulsive gag-making. Loosed from the comparatively reigned-in parameters of a half-hour's 'Goon Show', Milligan's manic (depressive) wit exhausts and repels as often as it delights, until it eventually burns itself out.

As 'The New Yorker's' Pauline Kael wrote about an earlier Lester film The Knack, but her argument can also be applied here, "If there are enough gags, perhaps the audience, panting to keep after them, will not worry about why they don't go anywhere." In other words, 'Too many jokes, Mozart.' Neither is the film helped by the presence of Sir Harry Donald Secombe CBE, the human equivalent of a housefly repeatedly dive-bombing a window pane.

The film's original tagline 'We've got a bomb on our hands' may have proved horribly prescient, but The Bed Sitting Room remains a haunting, dazzlingly inventive original, sporting more ideas, half-birthed or otherwise, than most movies of whatever genre, during its every minute of screen time. It is also strikingly photographed by David Watkin, who went on to lens The Devils, and ingeniously dressed by Assheton Gorton, the art director responsible for painting an entire South-east London street red for Blow Up.

That million dollar budget was certainly put to good use too. Against a dystopian backdrop common to every post-apocalyptic sci-fi from A Boy And His Dog to Mad Max Beyond Thunderdome, there are some unforgettable images and uniquely homegrown touches: a tube escalator ending in mid-air; two lovers dancing in a vast field of smashed crockery; the dome of a submerged St Paul's Cathedral, rising from the sea.

If the film's antecedents include everyone from Flann O' Brien to Samuel Beckett, its influence on those that followed is clearer still. A link-chain between The Goons and Monty Python, it is The Bed Sitting Room's fate to be remembered more as midwife, a Baptist to successive prophets of comedy, than as star in its own right.

All the same, it is fascinating to observe it going about its business at a very particular juncture on the comic timeline. At this point, the spirit of satire, personified by Peter Cook, is figuratively and literally floating above proceedings, while Spike's shrieking Goonery is still very much on the ground. Here's the man himself delivering a custard pie in the face, as a postman would deliver a parcel ("Just sign here"); here's his voice, clear as day, riffing through other characters with throwaway one-liners: "I hear the Pope's allowing contraceptives for all occasions, except during sexual intercourse..."

Satire will out, but it's a dismal victory in anybody's book. At time of writing, 2009, the British political system is busily imploding, with numerous politicians having been caught with their arms in the till, helping themselves to extra expenses for imaginary mortgages or using taxpayers money to clean out their moats. This, during an era in which the economy itself is in freefall. In The Bed Sitting Room, Arthur Lowe elects himself Prime Minister one morning, simply because he measures his inside leg and finds he's "very well equipped... I always knew my inside leg would lead to power".

Amid calls for a referendum on proportional representation, and with the democratic process under review, who's to say ministerial postings shouldn't be decided by the mathematical exactitude of an inside leg measurement? At least it wouldn't be open to interpretation.