The fly-on-the-wall mockumentary has surely run its course, but Trinny and Susannah: From Boom To Bust, succeeded in finding a little life left in the format.

The show followed the celebrated fashion counsellors and bosom gropers as they struggle to arrest a potentially terminal career decline. We witness a lucrative advertising contact fall through, their agent desert them, publicity stunts backfire catastrophically and their appearance at a golf convention fail to generate interest in their anti-fat underwear range.

Vanessa Feltz, David Furnish, Lulu and Prince Edward - I kid you not - are amongst the luminaries featured in supporting roles, but the programme stands or falls on Trinny and Susannah's performances, and they are pitch perfect. The pair pull off the difficult trick of playing the comedy whilst maintaining authenticity, and what could so easily have descended into self-indulgence proved a funny, poignant and strangely moving portrait of friendship in adversity.

An hour-long cobbled-together translation of a series of mockumentary webisodes that they posted online this summer, From Boom to Bust tells of the What Not to Wear duo's attempts to regain the limelight. The first 20 minutes are not bad, with Katy Wix giving good support, but the programme soon descends into sweary nonsense. "We used to be huge. We used to be something," opines Trinny in a Marlon Brando moment. Yet the biggest thing about her are those suspiciously plump lips. If the pair were really intent on deriding themselves for laughs, how could they have missed that trick?

Trinny and Susannah: From Boom to Bust (Channel 4) was the online spoof documentary about the fashion duo's attempts to relaunch themselves on the makeover market, retooled for the ancient medium of television. After Nigella, it looked like cinema verite.

It is a fairly gentle send-up of their respective personalities and collective public image. Susannah is an erratic boozehound - her long-suffering PA Gemma habitually has to run after her with forgotten items. "Susannah - phone! Lighter! Fags! PANTS!" - while Trinny is the more practical half, delivering intense speeches to camera while having a bum massage.

There were many nice touches. At meetings with commissioning editors, Gok Wan ("You know, that Chinese woman") is cited every time. Their new agent, Toby, brings along the clothes he wants them to use for makeovers: "I picked a few things out. How hard can it be?" It wasn't brilliant, but it was much, much better than it could have been, even allowing for the fact that the pleasure in watching Trinny and Susannah has always been that while they always took their tasks seriously (hence the spasms of frustration they endured in changing rooms up and down the land as recalcitrant, Widdecombe-shaped women refused to try on anything other than box pleats), they took themselves seriously not at all. All in all, a splendid palate cleanser after the heavily larded nonsense of Nigella.

Trinny and Susannah aren't in the same class as Nigella, more superannuated teenagers veering between strop and sulk than racehorse. But they're out of the same stable: they don't need second names; they're posh, of a certain age (46 and 48 respectively) and made their names on TV. They've also turned themselves into brands, and "do" humour, brilliantly. Readers might remember their fashion column in The Daily Telegraph's Weekend section, which, love it or loathe it, was a must-read for years. They took off into the fashionista stratosphere immediately afterwards with their TV series What Not to Wear, bullying members of the public into makeovers.

From Boom to Bust took up the story from there. It was a glorious spoof fly-on-the-wall documentary following the supposed collapse of their career. During this exquisitely funny offering, where small-screen setback was treated with the high seriousness of a Greek tragedy, every element of their career was mercilessly parodied.

We saw Trinny in a bath, covered in a hideous white facemask complacently discussing her diary. Cut to a taxi, where she was moving her colonic irrigation appointment so it would come after her "arse-slapping" slot with her masseuse (Think about it").

Cut to lunch with their agent, who told them not only had they failed to win a huge contract, but he was dumping them, too. Media names and celebrities appeared, as themselves, to mourn their decline: Dylan Jones (GQ editor), Lulu, David Furnish, DJ Neil Fox...

It got much worse, and much funnier. On to a golf and tennis trade show to promote their "magic knickers". "Susannah has a bit of a wobbly tummy," Trinny told the bystanders who had wandered up to their stall, hoiking up Susannah's dress to show how the magic knickers (deeply unflattering, flesh-coloured efforts) worked. Which is exactly the type of indignity they put their stooges through in their various makeover shows.

Will they, won't they climb back into the media spotlight? Perhaps they should get a makeover by Nigella. Now there's a thought.

Last Night's TV: Trinny & Susannah: from Boom to Bust

On the face of it, at least, they go in all guns blazing.

Is this a case of life imitating spoof?

It seems that appearing on I'm a Celebrity... Get Me Out of Here! is no longer the final roll of the dice for faded TV personalities looking to revive a dead career. There's also the self-satirising mock-documentary.

Spoof doc with the now washed-up telly fashion mavens, following them as they try to resuscitate their flagging career. Talking heads with Lulu, Jake Shears and Dr Fox add authenticity. Ditched by their agent ("When I first met them, they were Susannah and Trinny . . .") and out of TV offers, the pair start to peck at each other like irritable vultures. Their acting talent is genuinely surprising and they have superb support from Katy Wix as long-suffering assistant Gemma and Nicholas Burns as ex-agent Leonard. In terms of personality, this is how to look good figuratively naked.

With their crowns slipping, strident style queens Trinny Woodall and Susannah Constantine still have one has-been to make over - their career. The pair haven't chosen an obvious route for revival, sending themselves up in a warts-and-all mockumentary (originally shown in instalments online). Cameras follow them as they attempt to re-establish their TV profile. Some of it is hilarious. A drunk Constantine chain smokes her way through couples therapy with Woodall, and the desperate duo flog their range of underwear at a golf trade fair.

Earlier this summer, fashionistas Trinny Woodall and Susannah Constantine posted a series of spoof films on the internet about their supposed attempts to get back on TV. A mixture of improvised and scripted scenes, spattered with F-words, they've been edited into a one-off comedy. It's supposedly modelled on Curb Your Enthusiasm, but at times the screaming and swearing make it more like something a group of drunken and/or stoned students would produce. That said, you have to admire them for mercilessly poking fun at themselves. "We used to be huge," screams Trinny in-between colonics, while Susannah spends most of her time in a cigarette- and alcohol-fuelled haze. It's fun to star-spot too: Lulu, David Furnish, Neil Fox and even a bemused Duke of Wessex make an appearance.

Trinny and Susannah - remember them? The telly fashionistas were all the rage a few years back, only to fall into relative TV obscurity after an invasion by an army of new clothing experts, captained by Gok Wan. They're back tonight though, but rather than doling out clothing advice to Britain's worst dressed, they send themselvs up in this spoof film that's more like Curb Your Enthusiasm than The Clothes Show. You've got to applaud the pair for mercilessly mocking themselves, and keen eyed viewers will spot cheeky cameos from the likes of Lulu and David Furnish.