Tim Firth is fed-up with formulaic drama. "I'd like to submit a drama under a false name about the British Transport Police - the only part of the police that hasn't yet been dramatised - called Get Off, You're Pissed and see how far it got. It would be eight episodes of the police chucking drunks off trains." The depressing thing is, in the present climate it would probably be commissioned and become a big ratings grabber.

There is still a place for the type of drama Firth writes under his real name, however. His wonderfully quirky Preston Front - a sort of comic Our Friends in the North West - returns for a long overdue third series on Monday. With its sparkling array of original characters interconnected through the Territorial Army, it is very far from mainstream, by-the-yard drama. There's not a vet or a pathologist in sight.

Preston Front is like the white rhino of the TV schedules; it is that rarest of breeds, a comedy-drama that works. "When dramas are not formulaic, they do stand out," Firth declares. "When a treatment rests on a formula, what you get is limp drama. If I have a passion for anything, it's not for writing real comedy-drama, as opposed to something that's neither. You can't let yourself get away with it being either not funny enough to be a sitcom or not dramatic enough to be a straight drama. Comedy-drama is the holy grail of television. When it comes off, it outstrips everything else. The fact that you can still pick out shows like Boys from the Black Stuff and Auf Wiedersehen Pet shows how difficult it is to do.

"The problem is that comedy drama has often been used as refuge rather than a goal," he continues. "A lot of things have been thrown into the comedy-drama sin-bin. It's the equivalent of saying, `This drama isn't good enough for adults, let's do it for children.' My final aim is to make comedy-drama that's both, all the time. Like The Larry Sanders Show - when you watch that, half of you doesn't want to laugh because it's damaging."

Firth's other great skill is his deft conducting of an ensemble piece; unusually for a mainstream drama, Preston Front has no fewer than nine leading characters. "It's more fun to orchestrate something for nine instruments," he says. "With the group scenes, you can have a piccolo chirping away in the background while a big bassoon comes in and makes a huge impact every once in a while before going back to eating biscuits. The attraction of a big group is that gradually it becomes your landscape, the banter becomes your hijacks and car chases. Dialogue is king, and the muscularity of the drama can happily reside in one room. Look at Cheers."

The instruments in the Preston Front orchestra often clash, giving the drama a grown-up, fully-rounded feel. "All the relationships in it are defined by war," Firth observes, "but it's nothing to do with guns. Someone asked me, `Why are they friends, all they ever do is battle?' But in my experience, that's the foundation of friendship. It happens by the back door. The characters are at each other's throats one minute and sitting quietly talking about something else the next. In the juxtaposition, you say more than you ever can with lots of people hugging each other - which is the American concept of friends."

At the age of just 32, Firth is already a veteran of authored TV drama; he was commissioned to write the first series of Preston Front when he was only 25, and went on to pen both Once Upon A Time in the North and Money for Nothing. But he fears for the future. "It'll soon start to show that single-voice drama is gradually being squeezed out," he warns. "You can feel the ground underneath your feet appearing more and more shaky. The thing that really excites me is the single-writer series, something that has a chance to grow and become part of people's lives. I've absolutely no interest in setting up a series like Ballykissangel for other people to write."

A passionate, inspiring advocate for quality drama, he makes a final plea for commissioning editors not to patronise audiences. "Once you say that an audience can't cope with a certain tone, then immediately they won't. It's a muscle you lose. Drama should not be the sort of television you watch merely to decide where to go on holiday this year. Audiences are capable of going for stuff where they've got to do some work. If you say to them, `It's too demanding for you,' then where does it end? With Get Off, You're Pissed?"

A new series of `Preston Front' starts on Mon at 10pm on BBC1