The novelist and scourge of Catholic liberalism has a long hit- list. Photographs by David Gamble
To approach the murky kingdom of the novelist Alice Thomas Ellis, one must first inch one's way down Camden High Street amid the appalling traffic and peripatetic groovers who throng the thoroughfare, park in a ruinously expensive Pay 'n' Display enclave and negotiate past the Strongbow- swilling drunks of the Inverness Road market. Around you, skinny mongrels flinch away from their tough costermonger masters; bewildered alcoholics scour the abandoned end-of-day cardboard boxes, looking for unsellably subnormal efflorescences of green pepper and black banana. Around the corner, in fashionable Gloucester Crescent, the novelist's home looks under siege from scaffolding. A metal-barred gate with squawk-box suggests that visiting hours may be restricted. The garden looks uncared-for, the scullery (where her youngest daughter Sarah presses the buzzer to let you in), a picture of lampless gloom.

This unpromising territory has for two decades been the domain of two of the literary world's great eccentrics: Colin Haycraft, the late publisher who moved his company, Duckworth, to the Old Piano Factory in this street in 1972, and his wife Anna, known to nom-de-plumery as Alice Thomas Ellis since the publication of her first novel The Sin Eater in 1977. Between the raffish, real-tennis champion with his fondness for new-minted Latin epigrams and outrageous statements ("Only women can write novels, of course," he used to say, "only women and queers.") and his devout, maternal and eldritch bride, they came to epitomise north London bohemia.

Their garden parties were legendary affairs where you'd find Jonathan Miller, Angela Carter, Kingsley Amis, Oliver Sacks and a handful of denim- clad separatist lesbians rubbing along nicely together, and where two sips of Colin's fundador-and-Asti champagne cocktail would, mysteriously, render you deaf in one ear. Mrs Haycraft would survey the Bacchanalian throng like a stern but game headmistress, taking in a hundred strands of gossip at once and making discreet enquiries. "You know people here, darling," she said to me once, while gazing at a vast transsexual in a fright wig and PVC bustier, "Who is that man over there with the tits?"

Colin died in 1994 and, apart from not getting the newspapers delivered any more, "nothing," she says, "has changed. Most of the family's still here, so the peeling potatoes still goes on. Everything goes on in the same way, except that Colin's not here." We are sitting at the big wooden table beside her kitchen, with clothes steaming on a range on one side and a plaster St Francis of Assisi surveying us on the other. Other plaster saints, Virgins and the Redeemer himself pop up around the Haycraft house, just as the children (there are five) always seem to be drifting in and out, or their spouses or their own children. Mrs Haycraft is a welcoming hostess but today is irritated by the arrival of an interviewer, because she is trying to watch a Joan Crawford movie on afternoon cable TV ("It's what I do when I'm supposed to be writing, in common, I think, with every other writer in the world"). As a kind of displacement activity, she is worrying about a member of her family: "I've lost the bloody cat. I'm so worried. I haven't seen him all day. It's Basil, he's the light of my life. Oh where is he?" Her distress is clearly genuine, but also hammed up for effect; she sounds just like the heroines of her novels with their wise and beautiful apercus about life, endlessly undercut by existentialist melancholy.

From The Sin Eater and Birds of the Air, and the Booker-shortlisted The 27th Kingdom, to the Clothes in the Wardrobe trilogy and beyond, she has - as Alice Thomas Ellis - ploughed an exquisite furrow of black comedy shading into satire. Her new novel, Fairy Tale, returns to her favourite territory of rural Wales, where a young ingenue called Eloise sits sewing lace outside the cottage she shares with a boyfriend, becomes enchanted and steals a baby. The book tilts at all manner of fashionable New Age idiocies and post-feminist positions ("all that tripe") which accrete round the figure of Moonbird, a bogus visionary based on someone from the real-life Institute of the Creation of Spirituality in California. "I met some Californians and they were exactly the same," she says. "Very into Red Indians and angels. You know, they actually pray to be granted a parking space."

Everything else revolves round Eloise, the Ellis alter ego. "She's bored, you see," says Alice Thomas Ellis, as though speaking of a headstrong acquaintance. "She's pitched up in the country, expecting to commune with nature, and discovered nature doesn't commune back on the whole, that it's a one-way process. She doesn't want to admit she's made a mistake, so she thinks if she had a nice little baby to look after, then she wouldn't be bored any more. And she wouldn't, either," concluded the novelist with the air of one who knows.

Fairies, as the title suggests, lie behind Eloise's transformation; they come to her door in the guise of four besuited businessmen. "The Welsh are always seeing fairies, but not your usual ones with wings and so forth," she explains. "There's lots of different sorts. Some of their fairies are little tiny people with little tiny horses, richly clad. Some of them are like dwarfs, terribly ugly. They've such a range of fairies, you never meet two the same."

So she was using the traditions of Welsh folklore to power a narrative in which... "It's not just folklore," said Mrs Haycraft irritably. "Until very recently, they believed implicitly in fairies. It was only 30 years ago that one old farmer still put out milk for them. You'd get 'corpse candles' - when someone was going to die, you'd see the light flickering at the doorposts - and you'd see 'phantom funerals' go by, just above the hedgerow, and someone would die a week later and taken for burial along the same route." Had she seen one herself? "No, no, no, I never see anything." Did she believe in fairies? She considers. "Well, if all these things were simply a matter of marsh gas, of phosphorescence - where's it all gone?"

Note the body-swerve around the question of what she actually believes. Anna/Alice is a mistress of contradiction and shape-changing, as if she cannot bear to stay the same person for more than a few minutes at a time. Her voice changes as she talks, from an exquisitely liquid regal cooing to a curious metropolitan demotic, full of glottal stops ("Nah, there's no' a lo' o' tha' nowadays," she observed at one point) and expressions like "Gawd strewth". A devout Catholic, she is entranced by pagan gods. A stern moralist, she is a liberal dispenser (and consumer) of wine and fags. A strict fiction editor at Duckworth, and an advocate of tight narrative in her creative writing classes, she favours the see-what-happens approach in her own work. "I only wrote one book to a structure, The Inn at the Edge of the World, and figured out the whole plot before I started." She smiles, as if struck by what a good idea it might be. "It's much easier that way, like painting by numbers. But for the others, I just start with a character or an idea, and hope for the best."

Her advice to aspirant writers is simple: "I still think the best way to do it is to imagine you're telling a story, with all the digressions and Oh-by-the-ways and Did-I-tell-you-about-the times and No-tell-a-lie-it-was-a-Thursday." And then refine it, burnish it, rewrite it? "Oh," she said effetely, "I can't bear to do more than one draft. And it has to be quite short because then I have to put in all the insertions and I get into a muddle. I'm not really a novelist, you see. I can't see myself as a Lady Novelist."

She wrote her first novel, she says, out of irritation with the Catholic Church. It was written while her son Joshua lay in a coma. Nineteen years later, after the death of her husband, she turned her anger on the Church once more, in a well-publicised attack on the late Derek Worlock, Archbishop of Liverpool. Writing in the normally uncontroversial Catholic Herald, she accused his archiepiscopal shade of weakening the Catholic faith by his fondness for ecumenism, presiding over falling church attendances, plummeting vocations, obstruction, evasion and a mass of pastoral shortcomings. Hell followed. The nation's Catholic bishops responded with fury, the paper's deputy editor had to print a grovelling front-page apology (the editor, Christina Odone, had already resigned), the church's liberal following was outraged, and Mrs Haycraft's column was spiked ("I'd got bored anyway," she says. "I'm not really a journalist - I don't like this deadline lark"), Catholic traditionalists were up in arms and everybody boycotted the hapless Herald.

For those who know only her fiction - so cool, so silken and level-headed - or her party persona (she is an indulgent listener, a passive and smiling presense), l'affaire Worlock was something of a shock. It confirmed what had, outside the Catholic press, only been hinted at: that Mrs Haycraft is an unsmilingly extreme and implacable traditionalist about Catholic values, looking back well before the Second Vatican Council ("An utter nightmare; it led to endless, endless disobedience") to the days of absolute, all-or-nothing doctrinal rule.

She is appalled by abortion. She detests ecumenism: "You say, Why shouldn't a Catholic be able to take communion in a Protestant church? Because it's not the Body and Blood of Christ? So what's the point? And don't give me that stuff about a 'shared meal'. If you want one of those, go to McDonald's." At Sunday Mass in St James's Church, Spanish Place, she spurns all matey offers of "the sign of peace" and buries herself in a hymn-sheet.

Why had she focused on Worlock as the embodiment of her dislike? "Did you read what I wrote? It was frightfully measured. I wasn't attacking stupid old Worlock, I was attacking his results and what he'd done to lovely, lovely Liverpool. Rude? You should see the letters I got from people, saying what a nightmare he'd been, how he'd closed all the Catholic schools in Portsmouth, how he wouldn't respond to complaints about his priests but would just bounce along in his modernist heresy."

In the hallway, she showed me a large filing cabinet where she keeps her letters from the conservative faithful - along with updated files on the errant behaviour of a dozen Catholic bishops. This lady means business.

Did she regret the trouble she caused? "Good Lord, no - I've been struggling for years to get one squeak out of the bastards. Just one response. And then I discovered it's simple - you just attack one of them. Bishops hang together - they call it 'collegiality' - and they don't like being criticised. They demand obedience and I won't be. My first loyalty is to the Pope." A pause, as she executes a neat U-turn, "As long as he remains a Catholic."

Had the row come to the Pope's ears? "Oh, I think it must have done." Would she accept a rebuke from the Supreme Pontiff? "It would depend on how it was phrased." Oh come on (I said), if the Pope said you were wrong to criticise a senior Catholic figure after his death, would you think again? She considers. "I suppose I'd say, 'Well I'm very sorry, m'lud' - or whatever you call the Pope these days - but my conscience was my guide, as per good Catholic teaching, and I felt it was incumbent on me to say what I did."

Did she and Basil Hume? "I wish you wouldn't say Basil. Where is my poor cat." Had she met the Cardinal? "Hah! They said to me, 'Ooh, you're for it, you'll be up before the cardinal.' I said, 'Not a chance. His minders know I'd love to be before the Cardinal, to say a few well-chosen words." Didn't she like anything about him? "Oh, he's a good man, a good chap, but he's not a leader. He was so under the thumb of Worlock, they called him 'Cardinal Cushion'. He said, 'If there is a rift in the Catholic Church, I want to be a general on both sides.' Now, I don't know much about military strategy but..."

Was there a danger that all this Church Militant stuff would distract her from her art? "I don't feel like an artist," insists Mrs Haycraft. "I mean, although I cook, I wouldn't say I'm a cook. It's just something I do." OK, I say in exasperation, so far you're not a novelist, not a journalist, not an artist and not a cook. So what are you? "A Catholic," she says without a second's thought. "That defines me."

Upstairs, looking for a bathroom, I heard a tragic mewling from behind a door. Inside was Basil, enormously fat and pampered but temporarily stranded. I sent him down to his mistress. His reception was, believe me, more than his namesake would receive, should Cardinal Hume ever consider sauntering, tail erect, down the uncarpeted stairs of Haycraft Manor