To have your drama labelled "Our Friends in the South" while it is still in production is quite a burden for any writer to bear. But Tony Marchant, at the tender age of 37, seems perfectly capable of living up to comparisons with Peter Flannery and his multi-Bafta-winning epic, Our Friends in the North.

Marchant's Holding On, which starts on Monday, is one of the most ambitious dramas the BBC has ever taken on. A kaleidoscopic eight-hour serial, it features no fewer than 25 principal characters. In its grand range, it encompasses everything from snooty metropolitan restaurant critics through care in the community to massive tax fraud and random violence. With its sweeping vision and Dickensian use of London as a key character, Holding On has "event drama" written all over it.

In many ways, it is Short Cuts with a Cockney accent. Adrian Shergold, the director, describes it as "like travelling through London in a cab on a wet night. Every time you wipe the window, you see new characters."

Marchant is a tall man with close-cropped hair who was brought up in Bermondsey and used to spar at the famous Thomas-a-Beckett gym in East London. "At first Tony didn't know whether to be a boxer or a poet," laughs David Snodin, his long-standing producer. "He thought the theatre was full of poofs in cravats."

Over lunch on the location catering bus, Marchant outlines his aims for Holding On. "I wanted to write about London without describing it in detail," he explains. "You can't get an overarching take on it because it's too amorphous. The serial is about the fact that in a city the impression is that people lead segregated and fractured lives, they have an atomised existence. People can live in incredible proximity without knowing anything about one another. That was the genesis. What becomes intriguing is to see the interconnections between eight different stories. Some are shocking and some are revelatory. It's a bit like finding out about Estelle and Magwitch."

To paint on such a broad canvas is asking for trouble, and Marchant acknowledges as much. "You can be on a hiding to nothing when you say you're writing about London. You're bound to exclude far more than you include. There's also been a tendency to fix pictures of London, so that East End villains always talk in the back of Jags. That way of describing London has become hackneyed."

Snodin asserts that this series will be different. "Hanif Kureishi and Martin Amis have covered London well in novels, but it hasn't been covered well on screen since those `swinging London' films in the 1960s with Lynn Redgrave and Judy Geeson. Holding On is a love-hate letter to London. It's a wonderful opportunity for Tony to write about the city he knows best. The characterisation is immaculate. There's not a single character in this huge piece who hasn't been given great depth. Even a maitre d' character who came in for a day had an enormous back story."

Despite appearances, Holding On is not all doom and gloom and inner-city angst. The rabidly right-wing, bulimic restaurant critic (played with relish by Phil Daniels), for instance, provides many moments of light relief. At one point in the first episode, he rails against the fashionability of austere decor in restaurants: "What is it these days? If you can afford to eat out, it's got to feel like punishment."

"There was a feeling that if we were to reflect London as a whole, there should be some feelgood factor," Snodin confirms. "Some of us do have a good time here. London in the sun is still a very beautiful city. The last line of the whole thing is `I tried the country once: couldn't get a cab anywhere'."

Holding On is the kind of work that perhaps only the BBC could undertake these days. As Marchant, who has already scored critical knockouts with Take Me Home and Goodbye Cruel World, says: "Original drama is under threat. There has been a climate of BBC1 aping ITV, a panic about ratings-pulling drama, and a lot of second-guessing. The attitude has been `Anything in uniform will do, preferably dealing with animals'.

"Signature drama by writers with distinctive voices has been thin on the ground. But their impact is big. The sort of drama people will talk about for years has come from idiosyncratic people - Bennett, Potter, Bleasdale, McGovern, Flannery, Milne. Television shouldn't be ashamed of putting ideas across. People will remember those writers in years to come. They won't remember the content of Heartbeat."

`Holding On' begins on Monday at 9.30pm on BBC2