Adam Mars-Jones: 'My writing is like watching undercoat dry...'

Towards the end of last year, Adam Mars-Jones attended his publisher's winter party.

His reputation as a sharp and occasionally brutal critic preceded him: there was an awkward encounter with a writer whose book he had taken to task. "I said to my partner Keith, if my body is found in Faber's basement it will be like Murder on the Orient Express," he says. "There'll be at least 12 stab wounds and fingerprints everywhere."

Nevertheless, it was Mars-Jones's other job, as a short-story writer and novelist, that raised the most eyebrows. The publisher's party gave him the chance to launch his novel Cedilla, the sequel to 2008's Pilcrow. Mars-Jones had 90 seconds to introduce the work. The problem? Cedilla is 728 pages long. "It was like [Monty Python's] Summarise Proust competition," he says. "At least I was spared the swimsuit round."

Mars-Jones is making a habit of writing long books: Pilcrow may not be quite as magnum a magnum opus as Cedilla, but it still clocked in at 528 pages. Quite a departure for a writer whose previous work was infrequent and small: crystalline stories that examined the lives of gay men living under the shadow of Aids, or irreverent comedies such as "Hoosh-mi", which has the Queen catching rabies from a Corgi.

Indeed, Mars-Jones may still be most famous for the books he did not write rather than those he did: he was twice nominated by Granta as a "Best Young British Novelist" without having published an actual novel. (The Waters of Thirst eventually appeared in 1993.) "However much sour and knowing laughter it attracted," he says today, "Granta enabled me to survive in the marketplace."

We talk on a rainy day in Brixton, not far from Mars-Jones's south London home. Meeting him in person makes sense of Cedilla's polished verbosity. The 56-year-old could chat for England. In a conversation that lasts two-and-a-half hours, he glides effortlessly from the work of the Japanese film director Yasujiro Ozu via the renaissance of checked shirts and Nicole Kidman's downward trajectory to Margaret Thatcher's reformation of disability benefits.

His cutting, critical voice is present and correct. For example, Mars-Jones recalls discussing a "Big Read" project on the radio. "If raising awareness of books means Nicole Appleton telling us that the novel of Misery is even better than the film, then let's forget the whole thing. If it needs a member of All Saints to recommend Stephen King then we might as well give up. Because in order to sell it, it has become necessary to destroy it."

Mostly, however, Mars-Jones is courteous, funny and personable; a blend of self-confidence punctuated by occasional moments of self-doubt. "I have a bad relationship with writing," he says at one point, adding: "It's not that I don't think I have a certain amount of ability, but it's not easy for me to think of myself as a writer. If something isn't working, I assume it's total incompetence."

In this light, it seems miraculous that he finished one page of Pilcrow and Cedilla, let alone 1,250. What makes the achievement so remarkable is the peculiarly microscopic tale the saga tells: in short, the life and opinions of John Cromer. Having contracted Still's disease (a juvenile form of rheumatoid arthritis) as a child, Cromer spends most of Pilcrow under strict bed rest, and all of Cedilla confined to a wheelchair.

"I am only attracted to projects that are borderline impossible," Mars-Jones explains. "John Cromer's story is like some hideous bet. Let's write 2,000 pages from the least viable position there is. Let's put him in a wheelchair. Let's make him a faggot. And let's make him a strange religious nut. Go and chew on that."

What holds the reader's attention is the playfulness of Mars-Jones's prose and his astonishing narrative invention. Cromer spins pages of witty and closely observed prose out of love, religion, death and family, but also Kit-Kats, Bruce Forsyth and his mother's irrational fear of Tom Stoppard.

"This is a scrag end of a life, with no obviously thrilling features. It's like a slow cooker. If you take cheap cuts and want the fullest flavour, you need a low temperature for a long time. I didn't write something that's like watching paint dry – it's like watching undercoat dry. But it can be very liberating. The thing about difficulty is: when it begins to yield, it takes you to the most surprising places."

Nevertheless, Mars-Jones is aware that Cedilla's tempo, infinitesimal attention to detail and subject-matter won't be to everyone's taste. "I'm not stupid. I know that readers of Pilcrow asked 'Is he never going to get out of that bed?' I can't be surprised that people who are able to run for a bus wonder why they should inhabit the mind of someone who can't do anything."

Mars-Jones says there are two more instalments of Cromer's story to come. The next part, which is almost finished, will either be called "Umlaut" or "Caret". His ambition is that this most restrained of epics will transform his own reputation for good. With a characteristic combination of assurance and anxiety, he cites how Remembrance of Things Past revolutionised perceptions of Proust. "Everybody assumed Proust was a party-goer with nothing substantial to say. They thought he was an exquisite who cared about clothes. After André Gide rejected Swann's Way, he said that not recognising Proust's genius was the greatest regret of his life."

Whether Mars-Jones becomes a 21st-century Proust, or even a bona fide writer in his own mind, remains to be seen. But that uncertainty is both the risk of writing fiction, and its excitement. "The only books worth writing are the ones the world is not ready for," he says. "If everyone said, 'Cedilla is the book we've all been waiting for,' I would ask: 'What kind of people are you?'"

Cedilla, By Adam Mars-Jones, Faber £20

"I was no more than a stray eyelash which the unobserved world would never know it had shed, unmissed ciliary casualty, cedilla with a C to hang from. In some way this must have been what I wanted. I had made it clear from the start that it was important for me to confront the world on equal terms. Nothing else would satisfy me. I had waited a long time for the day when there would be no safety nets, and here it was."