I spent Saturday night dressed as Mike Leigh in a community hall in Stoke Newington. And I wasn't the only one. In fact, there were about 150 Mike Leighs, men and women, all kitted out in fake wigs and beards, jackets with lots of pockets in them, outdoor trousers and wedged sensible shoes, scruffy fleeces, old-fashioned checked shirts, woolly tops and anoraks.

No, it wasn't a reality TV show, nor was it a competition of any kind. The occasion was Mike Leigh's secret seventieth birthday party, and his sons, Toby and Leo, had issued strict instructions to come as Mike Leigh and be there an hour before he was due to arrive. The tension while we waited was unbearable. As Tim Spall glumly remarked, pulling on a strange cardboard beard and titivating his mutton chop whiskers, "This could all go horribly wrong."

It might have been Danny Blanchflower on This Is Your Life all over again, when the great footballer told Eamonn Andrews where to stick his red book and marched off in a huff, leaving an embarassing black hole in the night's television schedules (every episode of This Is Your Life was thereafter pre-recorded, never again broadcast live).  

Leigh was led in, blindfolded, by his sons while we formed a huge silent horse-shoe of bearded weirdos around him. Then, huzzah! The lights went up and an astonished, momentarily speechless Leigh broke into a yelp of surprise and delight and even failed to suffer the minor heart attack Spall was thinking of laying good money on. And we all sang "Happy Birthday."

It very soon emerged that Leigh was the best-dressed person in the place, by a mile. Almost dapper, he was, in black jacket and trousers, smart shirt (I've no idea where he thought he was going... The Bodyguard? The Ivy?) and soon giving notes on where we'd all gone wrong on our apparel and face hair.

The best Mike Leighs were definitely Alison Steadman and Janine Duvitski, the first affecting the middle-period khaki fisherman's two-piece combo from Milletts look, with a shaggy black wig and beard and a slightly over-pronounced sideways shuffle of a walk; the second, having researched the scowl and the shrug to perfection, trussed up in checks, off-green and grey trousers and a very dodgy jacket. Only one or two had risked the sandals on such a cold night.

The Leigh look can backfire badly, though. Indhu Rubasingham, for instance, new artistic director of the Tricyle, who worked on a theatre piece with Leigh at Stratford East years ago, resembled a member of Al Qaeda, fresh from the desert; and New York playwright Mike Weller, a bushy-bearded shepherd from the higher reaches of the Afghan mountains. Actor David Horovitch and composer Gary Yershon somehow seemed to be warming up for an amateur production of The Wind in the Willows.

Jim Broadbent simply said, "Well, he had it coming to him... all those years of working with him when, during the one-to-one rehearsal period, he's the only other person we've all had to look at all bloody day." (Broadbent's partner had coincidentally shopped at the same fancy dress store as my wife for a big bushy red beard that made her look like an Edwardian male impersonator and my better half like a strangely attractive George Bernard Shaw.)

The real Leigh is well into the rehearsal period for his next movie, only his second "period" film, about the genius Cockney landscape artist JMW Turner (played on the stage by Toby Jones in Rebecca Lenkiewicz's play at the Arcola a couple of years back), and starts filming in May.

Spall, naturally, is playing the artist, and other members of the cast enjoying a night off were Robert Portal, Sylvestra Le Touzel and Michael Simkins (Niall Buggy and Josh McGuire were otherwise engaged). "The research is never-ending," said Spall; "I've got a shelf of books about Turner that long - he spread his arms wide and added an extra yard or two - and that's not even the start of the stuff that's been written about him."

As the party progressed, old Leigh hands caught up with each other in various corners: Ruth Sheen huddling with Imelda Staunton, Duvistski with Brenda Blethyn, Robert Putt with Lesley Manville, all of them still tugging at their beards and shuffling around in their baggy pants and cheap shoes.

Only Richard Eyre seemed to rise above the imitation game, stylishly suggesting his beard with some white bobbles that suddenly appeared above his very smart designer black woolly scarf. Mind you, I couldn't ask him any more about it as he was unsurprisingly reluctant to engage me in conversation after my less than riotously enthusiastic notice of Quartermaine's Terms.

Broadbent wasn't the only Oscar winner in the room ("Stone the crows!"). There was also Lindy Hemming, who collected her golden doorstop for her costumes on Leigh's Topsy-Turvey in 1999. Other key behind-the-scenes colleagues included Kenith Trodd and Les Blair, both formatively cooperative producers in Leigh's early days making films for the BBC; David Aukin who commissioned stage plays at Hampstead Theatre (including Abigail's Party); film historian Colin Ford and London Film School boss Ben Gibson (where Leigh is an active board member); wizard theatre sound man John Leonard; and Leigh's current producer, Georgina Lowe, and ace publicist Jonathan Rutter.

Half the fun of the evening was in working out who the hell was who under the face hair. Actors Helen Cooper, Samantha Spiro and Paul Jesson took great delight in fooling people for as long as they could. Then a call to arms: a klezma band took the stage, people started dancing in circles as if they were in Zorba the Greek, and Leigh himself was born aloft on a chair, waving benignly and good-naturedly to his milling chorus of lookalikes. 

The moment had come for the cake: it was, of course, another Mike Leigh face job. It looked more than good enough to eat, and soon was being eaten, even though he'd blown all over it for about half an hour putting out his candles. The bar stayed open, the Turkish buffet ranged out simply in two dozen large plastic tubs on a long trestle table was delicious, and the night was young. Younger, at any rate, than Mike Leigh, though when I last looked he was turning into a leprechaun.