It's lashings of vodka for the Famous Five

Dick (played by Adrian Edmondson) and Julian (Peter Richardson), two members of the once youthful, but now distinctly middle-aged Famous Five, are at the bar of the Mad Cow Inn in Dorset, ordering a round of drinks. Dick, the dictionary definition of the phrase "arrested development", beams: "Wizard! There's nothing like an ice-cold ginger beer!"

"With lashings of vodka," adds Julian, dryly. This is a scene from the 41st instalment in the long-running series, The Comic Strip Presents. As you might have guessed already, this episode is entitled Five Go To Rehab.

The one-off special, which goes out on UKTV Gold at 9pm tomorrow night, is a spiffing spoof of all things Enid Blyton. Directed by Richardson, it marks the 30th anniversary of The Comic Strip Presents' first ever offering, Five Go Mad in Dorset, a note-perfect send-up that went out on the opening night of Channel 4.

The troupe, which began life in a cabaret show at the Comic Strip night above Raymond's Revue Bar in Soho in 1980, went on to make such memorable films as Bad News Tour, The Strike, A Fistful of Travellers' Cheques, and Detectives on the Verge of a Nervous Breakdown.

Co-written by Richardson and Pete Richens, Five Go To Rehab catches up with the Five – Dick, Julian, George (Dawn French), Anne (Jennifer Saunders) and Timmy the dog – 30 years on. The past three decades have not treated the quintet well. Julian has landed up at the Kneecap Rehab Clinic. He is also on the run from some bad guys after certain unspecified shenanigans in Equatorial Guinea. As Dick explains, "Apparently he was involved in some kind of voluntary work with a man called Mark Thatcher, whoever that is."

After her string of sugar-daddy husbands all met untimely demises, George has also ended up in rehab, while Anne is an embittered radical vegan. Meanwhile, Dick is the only one of the Five who remains completely unchanged. Stultified by his job working for a plastics moulding company in Dorking, he brings the old gang back together again to celebrate his birthday. Eager to recreate their gilded youth, he hires school uniforms, bicycles and ready-made picnics from nostalgicholidays.com. But their trip down memory lane takes an unexpected twist when Timmy is dog-napped.

The four main actors – Comic Strip regulars such as Rik Mayall, Nigel Planer and Robbie Coltrane also feature in the film – are relaxing between scenes in their school uniforms. The quartet are draped across the armchairs in the ornate drawing-room of the stern grey-stone mansion just outside Totnes, which is standing in for the Kneecap Rehab Clinic. The clinic has seen better days. Dusty mouse traps are strewn across the table, while a widely ignored sign – "Clinic Rules: No Drinking At Any Time!" – sits forlornly on the sideboard.

The performers are relaxed in each other's company – hardly surprising given that Edmondson and Saunders have been married for the past 27 years, and French and Saunders have collaborated since they first met at the Central School of Speech and Drama in 1978. The group take the mickey out of each other in the effortless way that only friends of three decades' standing can.

According to Richardson, "Getting back together has been great. We have been friends for such a long time that we are very familiar with each other. You immediately slot into certain roles and use a shorthand that makes everything very easy."

The director, 61, continues with a laugh that, "The other three started taking the piss out of me from day one. They show me none of the respect that I get from young actors who are keen to please me. This lot tell me the whole time that I'm crap. It's been horrible!"

The foursome can also banter for Britain. At one point, for example, 54-year-old Saunders assesses the current state of her character in Five Go To Rehab. "Anne has remained very uptight and slightly psychopathic. She's had a bad time. She has never married, but she's had several abusive relationships, which she probably encouraged! Things haven't really worked out for her. She's just come out of prison for setting fire to the nanny. We've all done it!"

"Absolutely," interjects French, 55. "How can that be considered remotely evil?" Later, Edmondson reflects on the silliness of permanently wearing a 1950s-style school uniform. All the same, he adds, "I wandered round Totnes this morning dressed like this, and I didn't seem out of place in the slightest."

"You know Totnes is twinned with Narnia, don't you?" French cuts in.

Revivals can be cringe-inducing – the old adage is, after all, "Never go back." However, Richardson feels this reunion is warranted because it has something to say about the dangers of attempting to rekindle a youthful idyll. The film majors on the theme of how it is not possible to live up to the idealistic expectations you harboured as a youngster.

He explains that, "Dick has had a very dreary life working in plastic guttering. The most interesting thing he can show Julian is a picture of his empty boardroom. Before he dies, he's desperate to reunite the Famous Five and relive the past. But when you go back to the past, you find things have always changed, and everything seems in the wrong place. It's like when you return to your childhood home, the house always seems much smaller than you remembered."

The idea of ageing is something that also preoccupies the actors. Edmondson, 55, reveals with a self-mocking grin that, "I made the mistake of looking at some rushes this morning. I wondered who that fat bloke was, then I realised it was me!"

But French chips in that age has not completely withered them. "The only thing we can't still do is get on the bikes so much. So we're in a car more now – let's put it that way!"

Richardson stresses that it was important for the comedy that none of the characters has thrived over the past 30 years. "If we said that the Famous Five had all been very successful, that they had all gone from strength to strength and become captains of industry and never been knocked back, that would have been boring. It is far more interesting to show these once cocky and moralistic people as damaged.

"It's much more fun if cracks appear as they try to maintain a semblance of what they once were."

Five Go To Rehab plays on the racism and sexism implicit in some of Blyton's work. Saunders muses that, "If you read Blyton now, the way the characters talk does seem funny. However, the joke now is that Anne thinks she's PC and picks Dick up for being racist. She knows that is wrong, and also thinks that meat is murder. So they have to have meat-free ham sandwiches at their picnic."

So why has The Comic Strip Presents lasted so well? Nick Smith, the producer of Five Go To Rehab, reckons that, "The Comic Strip brand has endured because there's nothing else like it – I would say that, wouldn't I? But I think it's true. Each episode stands as a film in its own right. They're all beautifully crafted. Peter is meticulous with his scripts, and they're all shot with the attention to detail of a feature film."

For all that, Richardson would be the first to admit that The Comic Strip Presents films have been hit and miss. "We try to do something different every time, and some have been stronger than others. "Perhaps we were more slapdash in the past. But when you do a long-running series, the strike rate will never be perfect."

So what's next? French is full of ideas: "How about 'Five Go to Positano and Eat Pasta' or 'Five Go to the Monaco Grand Prix' or 'Five Go to the Caribbean'?"

Richardson perks up. "The Caribbean is a good idea. I've got my bag packed already – I just need to write the script now! But on second thoughts, the film might generate resentment amongst viewers. They might think, 'I don't want to watch them having such fun, and wasting all that money in the Caribbean'."

Instead, they could always consider reuniting for a 60th anniversary film. "Absolutely," Richardson declares. "I'd love to make 'Five Go Mad in an Old People's Home'."

The characters would clearly not go gently into the night. "They would be great at disrupting the place," Richardson continues, "stealing people's medicine, going out on the town and getting arrested after a Zimmer-frame fight at closing time. I think we have just sold the next Comic Strip film!"

'The Comic Strip Presents: Five Go to Rehab' is on Wed at 9pm on UKTV Gold