Taking the BBC 1 Christmas night slot that was once occupied by Morecambe And Wise and then by Only Fools And Horses, the Christmas Special of Men Be- having Badly is expected to rake in more than 20 million viewers and thus establish itself as the sitcom of sit-coms. Not bad, when you consider that when the show first aired on ITV, it was dropped after its first series - and that the only thing it has going for it in terms of characterisation and storyline is the cliched concept of the battle of the sexes - in this case, two moronic men versus two not-terribly-interesting women. But since it started in 1992, then on ITV, Men Behaving Badly has struck more chords/rung more bells/fingered more pulses than all tepid rivals put together. Dovetailing nicely with the rise of bloke culture, the programme has discovered the precious alchemy of making something very new out of something very old - who would have thought beer and sex could sustain a series for seven years and make it the most award-laden sit-com of the 1990s? But it has always been more than just a 1990s update of Man About The House: the show takes the comic staple of gender difference and generally skirts around it via a series of recognisable stereotypes and a Route One approach to laughter.

Gary (Martin Clunes) and Tony (Neil Morrissey) are the two not-so-nouveau lads who share what used to be called a "bachelor pad". Charmingly child-like and happy as long as they know where their next can of Red Stripe/Sky Sports subscription/soft porn video is coming from, they are boys playing at being adults.

Enter, stage left, Gary's long-term girfriend, Dorothy (Caroline Quentin), a pleasant and clever person but ultimately a bit on the sensible and mousy side. Dorothy gets the post-feminist role of the "nagging wife" but with all the edges rubbed off. Not happy with her seemingly aimless relationship with Gary, she wants a display of commitment - marriage would be nice. Unfortunately, Gary is already married - to Tony.

Upstairs lives a far more idealised version of womanhood in the beautiful shape of Deborah (Leslie Ash). Over the past five years, Tony has slowly drooled his way into her affections and she has finally given in - or given up.

With everybody matched up, it's a case of Bless This House and we're On The Buses to successful sitcom wrap-up - except you can't walk away from these ratings too easily.

Eminently watchable and very funny, Men Behaving Badly works so well simply because it taps into every sit-com myth about the roles, functions and different concerns of men and women and puts a refeshingly modern spin on everything.

Gary and Tony represent every "buddy" relationship that's ever been and are perhaps the only two men on earth who actually call each other "mate" all the time: as in "alright mate?", "yeh, I'm fine, mate". The curious thing about all of this is that their relationship is more "female" than that of Dorothy and Deborah in terms of mutual support, emotional reliance and blind loyalty.

And while Gary and Tony are mirror images of each other, the women characters couldn't be further apart - Deborah represents the sort of woman most blokes want to get out with while Dorothy is the sort of woman most blokes actually go out with. Actor Martin Clunes, who has had a very successful theatre career and is also a director/producer in his own right, has so skilfully realised the part of Gary that, he says, it's a full-time job explaining to people that he's not like the screen character - at all.

"Gary is just someone who missed out on the usual assimilation of information that matures you," says Clunes in his slightly posh real-life accent.

"He just didn't pick it up and he's hung on doggedly to every misconception he had of everything, especially women - and no, I'm nothing like that myself."

It's got to the stage where Clunes feels sorry for his emotionally illiterate, sex-obsessed character: "Gary is continually failing, he's pretty pathetic and I've no idea, myself, what Dorothy finds attractive in him - but I do wish they'd get married."

Neil Morrissey, whose parents come from Ireland remarks that "because of Tony being a lad's lad, people expect to find me either drunk or drinking and unfortunately I do get drunk sometimes, so people do see me drunk and that's where the similarity starts. And people like to buy you drinks, which can be dangerous." Does he find the Tony/Deborah relationship plausible?

`I think they had to happen. He's worn her down and I think that's actually one of her lines in the show. Something had to happen. If I had to give Tony some advice I would say to him "wise up son", "get a job" and "earn a living". He's done a bit of life but he still seems naive about things: he needs to sort himself out, get a job and get a haircut."

Caroline Quentin takes a more jaundiced view of her character Deborah, saying: "I don't think anybody would put up with Gary, would they? People say she's very patient and puts up with an awful lot, but it's a sitcom after all - and life isn't like a sit-com.

"I really don't think there's a lot of me in Dorothy. Obviously there are a few similarities but she's much more dogmatic than I am and she's quite scary, I think. I hope I'm not that scary."

She professes herself well pleased with the way the scripts have developed over the years: "Dorothy used to be a comedy nurse in the beginning but now she's more rounded. You've seen her and the rest in a lot of different situations, emotional and otherwise. I think all the characters have grown with the series and become interesting. Tony's become much more eccentric - he's mad really - and Dorothy's become more human, which is nice to play."

In the series just ended, Dorothy and Gary had planned to marry but put it off - what advice would she give to her character about the relationship? "I'd tell her to travel, alone. Go away and see the world and then come back and see if you really want to be there.

"I think she will marry Gary at some time. I think they genuinely love each other and they make each other laugh as well. He is ghastly but also sexy and funny and all those things too. Loveable, very sweet. He's adorable . . . and ghastly."

The tension of the Tony/Deborah relationship has helped sustain the series, says Leslie Ash, who plays Deborah: "I think the public liked the fact that they never quite got there and that was the substance of their relationship. It was so much part of Tony's character - as in, he messes it up at the last minute. But there was too much of Deborah having a boyfriend and Tony getting jealous and he would always end up embarrassing himself. I like the idea of the writer (Simon Nye) putting us together, because we get much nicer scenes to do and I am very much part of the whole situation now, rather than just being a neighbour. It's now a series about two couples."

But how long will that survive? Tomorrow's special features Gary attempting to cook Christmas dinner for Dorothy while Tony persuades Gary to lend him money so he can buy Deborah expensive presents. This quickly changes to a liquid dinner and terrible presents that nobody wants - and then an argument starts . . .

The 45-minute Men Behaving Badly Christmas Special is on BBC 1 tomorrow at 10.20 p.m.