Deborah’s pride and joy

At home in Hampstead... Deborah Moggach. Picture by Polly Hancock

Deborah Moggach is looking tall tanned and toned, as befits the screenwriter of Pride and Prejudice, a glittering star-studded production with Keira Knightley as Elizabeth, Brenda Blethyn as Mrs Bennet and Donald Sutherland as a surprisingly sexy Mr Bennet.

Despite its Hollywood names, the film, the first of Jane Austen's best loved novel for more than 50 years, has been dubbed the 'muddy-hem' version, for having the courage to take a more honest look at the messy realities of middle class 18th century life than we are used to from our normal viewpoint from the sofa on a cold Sunday evening. Out goes Darcy's heaving chest, and in come bouncy dogs and puddles. Out go beautifully cut scenes of repressed needlepoint and bleached muslin, and in come gabbling girls with red noses desperately dying ribbons in beetroot to get the look for yet another ball they can't afford but dare not miss.

Keira Knightley makes an exquisite Lizzie Bennet, but thanks to her lack of make-up and her unruly curls, one we can believe in and truly care about. Matthew MacFadyen makes a darkly brooding Darcy, a less instantly steamy hero than Colin Firth, who stole the show in the now 10-year-old television version, but simmeringly unattainable all the same. If anyone were to prise the Quality Street tin away from the period drama it's Deborah Moggach.

Her home and surroundings have a tumbledown carefree beauty which epitomises the muddy hem version of Regency Hampstead. You'll find the Merchant Ivory version down the road in Primrose Hill with Sienna Miller. Deborah's muscles aren't the work of a personal trainer at dawn (unless of course her social life has taken a turn she hasn't told me about), but the result of her religiously regular walks on the Heath and cycle rides round Hampstead on her boneshaker of a bike. Her house was built in 1815, but unlike its neighbours, whose lawns are manicured to within an inch of their turf, you swish up to her front door along a brick path lined with black-eyed Susans, columbines and discarded pewter watering cans.

The only sign of modernity is yesterday's empty milk bottles waiting on the doorstep. Once inside you find yourself squinting inside a dark oak-lined hall, with plenty of room for all the Bennet sisters to eavesdrop on you from the staircase above. Below stairs is the kitchen, which leads out to the kitchen garden and henhouse, a perfect retreat for Lizzie to escape her squabbling sisters, especially when the hens are feeling sociable.

Mr Bennet would find a comfy corner in her wood panelled study at the top of the house, though he might find the four-poster bath a bit disconcerting. Even the studious Mary would never run short of books in her sitting room, heated by a real fire in winter and candle lit in summer.

Director Joe Wright could have filmed practically all the home life of the Bennets from her kitchen and garden and saved a fortune on location bills. Getting to the truth of what life must have been like for the Bennet family was what was uppermost in her mind during her writing of the screenplay, which is why she is so delighted with the finished film, with its overcrowded rooms, muddy entrances, and dung heaps, describing it as "breathtaking".

But she is refreshingly candid about the jumble of coincidences and business decisions that brings a film from conception into life. She had her eye on the young Joe Wright as director herself and put a word in for him.

"I'm sure I wasn't the only one, but I still think he owes me a drink," she said, giving one of her huge, generous laughs. The beauty of having Joe Wright as director, who had never seen the television adaptation or even read the book ("Lots of boys haven't read it," she explained) was, she said, that he came to it completely fresh with no preconceptions about people sitting silently round the edges of the room playing tense games of patience.

"He's very truthful, he's very much part of the world of social reality television. I wanted it to be very truthful with muddy hems and dung heaps, because these things mattered.

"The Bennet family was in a very precarious situation, with five unmarried daughters and the ghastly Mr Collins waiting to inherit the house. It wasn't just a fluffy social comedy," she said, adding that the reason the book was so funny was that the comedy arose from real pain, the real prospect that unless Mrs Bennet did her stuff and found suitably wealthy respectable husbands for her daughters, the consequences were dire. "And we see those consequences with Charlotte Lucas, over the hill at 27, who makes a conscious decision to marry a physically repulsive man she doesn't love because the alternatives are even worse - total dependence on her family, not being able to go anywhere unless someone else is going too, or a job as a governess, how grim is that? Mr Bennet was a gentleman farmer, although they were very middle class and had servants and a big house and a carriage, they were a very provincial, old fashioned rural family."

The contrast between the genteel but down at heel Bennets and the glamorous, fashionable and wealthy new arrivals at Netherfield was crucial to the plot of the book but something she feels had become blurred in the previous film (starring Laurence Olivier in 1940) and the television adaptation.

Unless we see the difference in the two families' circumstances, the boisterous overcrowded assembly dance in the village where Darcy is so devastatingly rude about Lizzie, we have no possible understanding of Darcy and his London crowd. "Of course he would be bored stupid there," she said, adding that, to be blunt, the Bennets wouldn't have been the most fascinating companions in the world. "They couldn't be, there was nothing for them to do but visit their neighbours and endlessly trim their hats."

The beauty of the work lies in its timeless obsession with family life. "Those sort of things never change - mothers are always embarrassing, we all want to see our daughters settled." Austen equally understood the powers of attraction "She understood the sexual charge of antagonism and quarrels. We don't fall in love with our best friends," she said.

She loves Austen, but not excessively, which allowed her a coolness and distance which seems to have stopped her work being too reverential, unlike some.

"Those Jane Austen people are so nerdy" she said, describing some critics' trainspotting obsession with period detail. "But just because they were a Regency family didn't mean everything was exactly of its time - they were very old-fashioned, like me," she said, waving a hand round her sitting room. "This is the 21st century but everything in this house isn't exactly 21st century." Apart from yesterday's paper I hadn't spotted a single thing that was. Writing the screenplay involved, she said, turning Austen's own dialogue from paper into drama, as if it were the simplest thing in the world, keeping the best lines. Many of which belonged to Mr Bennet such as his "You have delighted us long enough" to the unfortunate but accomplished Mary, and sacrificing some of the best loved. Try as she might, she couldn't put the words of the best-known lines in literature into anyone's voice but Jane Austen. So what happened to: "It is a truth universally acknowledged..."

"We cut it" she mouthed, half horrified, half delighted at the sacrilege. Until Pride and Prejudice came her way Deborah herself was best known for her novels, deceptively well-crafted social comedies like Ex-Wives, until she hit the mother lode with Tulip Fever, her seductive tale of a romance which blooms in the hothouse atmosphere of the 17th century Dutch bulb trade. She came within a whisper of Hollywood-style fame last year when Jude Law and Keira Knightley were to star in the film version of Tulip Fever and were days away from filming the first scenes when the chancellor Gordon Brown closed a tax loophole which made the whole thing commercially unviable and the production, not to mention several lorry loads of tulips, were scrapped.

Meanwhile Deborah was trundling away in her bedroom reworking her screenplay of Pride and Prejudice. She is thrilled with the finished film, but modest about her own part in it. "Most of the dialogue was already there," she said. She didn't do too much hanging round the set but admitted to a quickening of her heart when Donald Sutherland took her by the hand, praised her screenplay to the skies and led her round a potato field in Lincolnshire to meet the extras. And she wouldn't have missed the premiere in Leicester Square for the world.

But the real excitement will come when she sees the film properly, at the Screen on the Hill with her friends. She doesn't mind admitting that she will be hanging round the bar eavesdropping as she did in Liverpool when she had a play on there. She doesn't recommend it, but she can't help herself. "They're never talking about what they like about it or what they think will happen next - all you hear is 'Can you get me a gin and tonic while I go to the loo'."

Should Pride and Prejudice open some doors for her she will be the first to celebrate, but she won't be taking the first limo to Hollywood. "I wouldn't live there for a million pounds," she said. "I'd miss everyone too much and I'd miss my house and my dog." Not to mention her hens.

Meanwhile she's too busy worrying about not having a new novel in her head and planning the rest of her day. "I've got an acapella audition at the City Lit later, then I'm having tea with a prostitute," she says, pressing a box of this morning's eggs into my hand: "Laid this morning".

p Pride and Prejudice opens today at cinemas all over London. Deborah Moggach will be introducing the film at a special screening at The Screen on the Hill, Haverstock Hill, Hampstead on Sunday. The screening is to raise money for the charity Connections, part of World Jewish Relief, to raise funds for student scholarships in the former Yugoslavia.