It began with a shaggy, Ancient Mariner figure dragging himself along a seashore. Its jaunty credit sequence intertwined cartoon flowers and the photographed heads of Victorian grandees.

It introduced viewers to Arthur "Two Sheds" Jackson and the Funniest Joke in the World – and it was the start of a five-year love affair between the British TV audience and a world of cosily surreal humour. Monty Python's Flying Circus first hit the airwaves 40 years ago, on 5 October, 1969.

The timing was awful: it went out late on a Sunday night, in a slot formerly filled by a religious programme. But rumours of its surreal brilliance spread along the schoolboy grapevine, catchphrases ("And now for something completely different," "Bloody Vikings") began to appear in newspapers, and mildly satirical visual gags, like the Upper-Class Twit of the Year Contest or the Ministry of Silly Walks, were greeted as though they represented mad anarchy.

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By the beginning of the new decade (programmes 12 and 13 were screened in January 1970) the nation was hooked on the new cult of Python: men with knotted handkerchiefs on their heads, shrill-voiced housewives in blue rinses, huge cartoon feet descending from the sky to squash the action, ancient footage of WI ladies applauding, stiff newsreaders and pretentious arts-show presenters, mendacious shopkeepers, and intrusive brigadiers closing down sketches with the words "Too silly..."

But was it actually funny? As with The Goon Show before it and The Fast Show after it, Monty Python established the taste by which it was judged. It was different. There were no punchlines, no scenes that ended with close-ups of a discomfited face, no anchorman, no musical interludes, no studio audience and no laughter track. Instead, a mix of cod-interviews, cod-documentary and cod-information was stitched together by Gilliam's hilarious bloated animations and naked Victorian cut-outs.

Watching the first episode now, I can recall the delight with which (aged 15) I watched the procession of unexplained bizzarerie on the screen. The hairy old man of the sea struggling 100 yards to utter the word "It's...". The squeals of dead pigs as they're squashed one by one. Mozart presenting a series on Famous Deaths ("We start with the wonderful death of Genghis Khan, conqueror of India.") The Italian lesson in a class of noisy Italians. The interview that gets nowhere because the interviewer is distracted by his guest's nickname. The cross-cutting between a Grandstand-type TV studio and roving-mike correspondents waiting to encounter Pablo Picasso executing a painting while riding a bicycle on the A272. The conceit of "The world's funniest joke" which makes all who hear it die laughing, and is co-opted into the Allied cause in the war...

Inevitably, there's some distance between the memory of watching the first Python and the experience of seeing it now. Terry Gilliam's surreal animations are still a delight and haven't aged a millisecond. The logic by which scenes are linked (by blackboards, signposts, bits of elderly artwork) still seem the work of fertile imaginations, while it's hard not to laugh at the subversion of po-faced TV presenters. But after that – goodness, how tired it looks. The pacing is all wrong: after the rapid-fire early sketches, the "Funniest Joke" section outstays its welcome by several minutes, and leaves you frustrated that the joke itself is never translated from German. Did they expect the viewers to understand it?

The suspicion that the writers were being a touch elitist is worsened by the Oxbridge smart-aleckry on display. Would any comedy writer today name-drop so many historical names, confident that the audience would be dead impressed? Richard III, Marat, Jean d'Arc, Lincoln, Edward VII, Nelson, Mozart, Kandinsky, Braque, Mondrian, Chagall, Ernst, Kokoscha, Schwitters... Gosh, well done, chaps, for having a nodding acquaintance with French and art history -- not that it's being used for any actual humorous effect.

Modern viewers might also raise an eyebrow at the underlying homophobia: the arts interviewer who wants to call his film-director guest "sweetie", "sugar plum" and "angel-drawers" has a moment with another arty poseur. "Never mind, Timmy," says the latter. "Oh Michael you are such a comfort," says the former. You can hear the sneers coming off the writers like cologne: "These guys are pretentious. And they're poofs."

Looking back, I suspect it was the lovely randomness of the show – the sense that anything could start or stop at any time, or be interrupted by a giant foot or a human hand – that most delighted my generation in 1969. And we loved the individual performers, especially Cleese and Palin. But a lot of Python was stuck fast in the public-school-Oxbridge ethos, the comedy of the schoolroom, the naughtiness of playing foolish japes on figures of authority. It liberated us all from the tyranny of the punchline – but it may be time to stop thinking that the liberation was a revolution.