Whenever the history of the UK’s live comedy is written, the Comedy Store rightly takes centre stage as the venue which first opened its doors to alternative comedy and which continues to be one of the best stand-up rooms in the country.

But we wouldn’t have such a thriving scene without the myriad of other clubs which quickly came in its wake to form the comedy circuit across the UK. Local operators such as XS Malarkey in Manchester, The Stand in Scotland, and now-defunct clubs such as the Meccano or Earth Exchange in London. Formed in the image of their promoters, these grassroots venues became a proving ground for comedians and a cheap and varied night out for punters as the scene became a business and then an industry.

The Oldest Comedy Club In Britain, a new documentary from comedian turned film-maker Joe Bor aims to put one such venture in the spotlight.  Downstairs at the King's Head was set up in a pub basement in Crouch End, North London, in 1981, two years after the Comedy Store, where it has been ever since. That it has never moved allows it to make the titular claim, whereas the Store has had three homes.

Bor and his contributors may be guilty of overplaying the venue’s significance – ‘when you begin this is the big one,’ Seann Walsh asserts early in the film ‘it’s where you felt you had progressed from open spot to actual comedian’, though surely there are many such gigs that could be considered similar career landmarks.

But there are many things that make Downstairs at the King's Head  special, starting with its low ceilings, intimate layout, brick wall  backdrop, and smoky atmosphere, back in the day. ‘This place is what I thought a comedy club would be like,’ says Stewart Lee of his first time there, echoing what many other comics say on camera.

It attracts an audience of appreciative locals. Crouch End is in a Tube blackspot, so likes to think itself separate from other parts of London. ‘I feel I can take more risks here,’ Stephen Carlin says. ‘So I think a lot of like newer routines have been born here.’

However, the main asset is not bricks and mortar but Peter Grahame, who established the club with Huw Thomas, an actor and performing arts tutor and for a long time the club’s resident compere.

‘He gets all the simple things right,’ comic Alan Francis says. ‘Good sound system. Good lights, focus in the room always employs a good compere. Very supportive. Very funny as well. He's got a great sense of humour… he’s one of the most supportive people in the comedy business.’

Miles Jupp says the venue ‘nicer than other comedy clubs – that’s a fact’ which is down to Grahame – though as master of the domain he is no soft touch, giving unfiltered feedback to acts and insisting they call him during a strict time window to get onstage.

For newcomers, that is, however, a very equitable process: ring and you’ll get the next slot on the Thursday try-out night. ’The conveyor belt of dashed hopes was always fun to see,’ Lee recalls. But if you do well, you’ll progress to Sundays and then weekends.

In the venue, there’s a small office/green room to one side, which is Grahame’s inner sanctum. The unspoken etiquette surrounding this sacred space has become the stuff of legend as only those comedians who have proved themselves may enter – and no one will ever tell them when that time has come.

As a film, The Oldest Comedy Club In Britain, is a straightforward affair, with a number of comedians including Phil Nichol, Nick Helm and Roisin Conaty, eulogising about their time there, while Grahame and others reminisce about the acts who have been through the doors – the good, the bad and the weird.

Hugely affectionate in tone, the documentary has the slightly elegiac feel that we won’t see the likes of Downstairs at the King's Head again, especially as health concerns mean Grahame is likely to start taking a back seat before too long, inevitably spelling a change in the character of the place.

In a broader sense, regular club nights can seem like a scene on the wane across the board, under pressure from below by too many cheap-and-cheerless open mic nights, and from above from comics quickly making the leap to solo shows, eagerly booked by arts centres as a more economical bet than a theatre show. But the likes of  Liverpool’s Hot Water or London’s Top Secret have shown clubs can still thrive.

Bor last year captured the lure of club comedy through his intimate portrait of one of its best exponents in Ian Cognito: A Life And A Death On Stage. And here he adds to the mythology with a profile of the club that gave him his first ever gig, which brings an inevitable personal nostalgia to the documentary, and which represents just one manifestation of the haphazard scene.

And there are a few fascinating titbits, too, such as revealing the patch where the painted backdrop has faded away as nervous newbies back away from the spotlight until they can go no further.

Eighty-five minutes is probably too long for the film, with repetition that an hour-long version would exorcise. But the film serves as a tender portrait of idiosyncratic labours of love that built Britain’s vibrant comedy scene, so perfectly personified by Peter Grahame.

• The Oldest Comedy Club In Britain is being screened at the Art House Crouch End at 8pm tonight, at 3.30pm on Saturday and 7pm on May 3 and May 8. All screenings are followed by a Q&A with Bor.