'I never kid myself I'm going to be a Tom Cruise or a Johnny Depp but it'd be better not to be too grotesque.' Actually, Stephen Fry is looking rather good this lunchtime. He's given up smoking and he's slimmed down. His jowls are slightly firmer, his cheeks a little less plump. 'I have lost a bit of weight, yes,' he says. 'Thank you for noticing.' He's been filming for a forthcoming ITV drama and is growing tired of seeing the full-fat version of himself on screen. 'I wouldn't call it dieting, I'm just being a bit careful about what I eat.'

Actually, he's keeping an eye on how - not what - he is eating. Fry's ally in the battle of the bulge is Paul McKenna, with whom he has become friends after buying a copy of the super-cheesy stage hypnotist's weight-loss book. 'I just came across it. It's called I Can Make You Thin, and I thought: "Of course you fucking can." But I read it and it actually made enormous sense. He doesn't look at what makes people fat, he looked at what makes people thin. He said: we all know people who don't seem to put on weight, what's different about them?' What is different, says Fry, is that they eat slowly and they stop eating when they don't want any more food. The trick is to eat when you're hungry - but only when you are - and to put your knife and fork down between mouthfuls. It's good no-nonsense stuff. No faddy food plans, no lame excuses for being a lard-arse.

'All this stuff about glands. It makes me think of Peter Cook. I was on holiday with him in Egypt. We were ploughing through the papers and there was this photograph of Elizabeth Taylor at her most blown up and her spokesman saying it was down to her "glands".'

Fry launches into a delicious impression of his friend and hero: 'Yes, poor woman!' he squawks in Cook's voice. 'Elizabeth would be sitting in her suite at the Inn on the Park, quite casually watching the three o'clock at Haydock, and suddenly her glands would pick up the phone and order a tray of eclairs and a bottle of Courvoisier! No, she'll scream! But it's too late! The order's gone through!' Resuming his standard delivery, Fry explains that Cook's fantasy ended 'with Taylor in the bathroom, trying to lock the door against the glands, while they smash their way in and force the food down her throat.

'It was such perfect satire and typical Peter. It put paid to that nonsensical bollocks that your body can manufacture fat out of nothing.'

Actually, today Fry retains a constant grip on his cutlery and, by his own admission, continues eating long after his hunger is sated. There are two reasons. Firstly, he is in a bit of a hurry (McKenna's method only works when you've got plenty of time to eat) and secondly, we are lunching at Strattons, in Swaffham, Norfolk, Fry's favourite local restaurant. This is where he comes to celebrate family birthdays and when he doesn't fancy cooking at his home a few miles away. Strattons, he says, has 'neither made the terrible mistake of trying to be a metropolitan restaurant in the middle of a country town nor of trying to be a ghastly idea of what an English country house should be. That's why it works so well. And the food is excellent.'

But I'm a bit worried about what he makes of the wine, which has been chosen on our behalf by the proprietor. Though Fry is perfectly polite about the bottle of English white on the table, a Thelnetham which comes from just over the border in Suffolk, it's unlikely he'd have chosen it himself. A few minutes ago, before we sat down, he was telling me of the time he ordered a £900 bottle of wine while being interviewed by a journalist from the Daily Telegraph. The result? A fine lunch but tears from the writer when he saw the bill, a dismayed phone call to Fry from the newspaper's editor - and, more entertainingly, a few words from Conrad Black, the Telegraph's proprietor at the time. Black, a man of notoriously expensive tastes, was, of course, much impressed by Fry's chutzpah.

At home, the actor and writer's tastes are a little less extravagant. Fry's partner, Daniel Cohen, 'cooks everything. He is very good indeed: he does Simon Hopkinson's wonderful roast chicken, with his own variations; lamb with anchovy, which is very good. And I call him the world's premier saladeer: he is an incredible maker of salads and dressings. He'll find a pomegranate, carrot and a piece of bread and he'll somehow make a salad out of them that is just delicious.'

Fry's own efforts are rarer, though he is still proud of the pumpkin pie he cooked for his friend Hugh Laurie for Thanksgiving last year. He'll also sometimes creep into the kitchen in the dead of night: 'If I can't sleep - at the moment I've been sleeping well, but there are periods when I have insomniac episodes - I tend to go down to the kitchen to make mayonnaise, which I love doing. I find it's exactly the right blend of concentration and repetition. The satisfaction of having this beautiful velvety stuff that you've just made out of oil and egg yolk is very pleasing.'

If Fry, 49, has a love of fresh, simple ingredients, it's because he picked it up as a young boy from his family cook and housekeeper. He is still in touch with Mrs Riseborough, more than 40 years after she started working for the family a few miles from where we are having lunch. With Mrs Riseborough's crucial input and the distance of time, Fry's Norfolk childhood reads like an early draft of an Enid Blyton story.

'It now seems very old-fashioned but, of course, nothing does at the time. My parents had gardeners and a cook and the house had a large Victorian garden that produced everything, in the way that those sorts of houses do. So the gardeners would come to the kitchen door and give the cook the vegetables for the day. There was asparagus, peas, marrows. It was wonderful.'

What didn't come from the grounds was delivered to the door. 'The fish man came on Wednesdays on his horse and cart; the butcher, Mr Tuddenham, came on Tuesdays and Thursdays, I think; and the bread man three times a week.'

The comforting rhythm of life was reinforced by Mrs Riseborough's love of routine. If it was Thursday, it was treacle tart for pudding - though there was some variety. 'My brother Roger liked it with Cornflakes on top and I liked it with breadcrumbs - so that'd alternate.' Other days saw egg custards, suet puddings, apple jellies.

Mrs Riseborough - 'Mrs Raspberry', to Fry's younger sister, Jo, 'Mrs R' to his mother - taught Stephen how to bake, a skill he still values, and helped instil a love of all things English which today expresses itself as 'a conscious relish of all the great symbols of old Englishness - I love St James's, Lord's cricket ground, the Garrick and the Beefsteak and getting a good suit made in Savile Row'.

But there was another side to his family. Fry's mother, Marianne, is the daughter of a boisterous Hungarian Jew who came to this country just in time to escape Hitler.

'All my mother's family, apart from her two sisters, were from Israel, New York and Europe. And they did this thing that English families just don't do - they talked about food. And they loved it ...' He slips into a mittel-European accent: 'Fantastic! Oh, mein Gott! Look! Vunderful!

'These days we all talk about food and think it's natural but in England food used to be talked about as if it was excrement, as if it was something to be ashamed of, something that you don't talk about at table. What was so enchanting - and embarrassing - about my family is that they always talked about food and they loved it.'

How Jewish was the food? 'Oh they were never kosher. They were not in the least bit religious.' One dish his grandfather was particularly proud of was brawn - pig's head with jelly.

Marianne's side of the family had twice faced shortages of food. 'After the First World War, Vienna was in total ruins - even people from good families were starving in the streets. And then of course it happened again to Jewish families after the rise of Hitler.' His mother's relatives 'were people for whom a crucial part of their lives had been spent as refugees, beggars and camp internees. Therefore the relative richesse even of 1960s England, which to us would look dreary beyond belief, was something to be celebrated every day.'

Not that Fry himself did much celebrating. If his childhood sounds wonderful to outsiders, to Fry it was impossible to see it like that.

'It was life. It hardly seemed delightful at all, no.' He felt stifled and isolated by the rigidity of the countryside. 'I would visit friends who lived in towns and cities and just be staggered by the freedom that they lived under. They could just say, "I'm going out to play" to their mother, knock on someone else's front door, and their mother would say: "Oh yes, he's upstairs in his room." Whereas when people came to our house, tea was laid out in the drawing room and you'd sit on the sofa and talk. Because you were miles from the nearest person, it would be an event if someone was coming round.'

When he was diagnosed with manic-depressive illness 11 years ago, Fry was suddenly able to understand why his early years were populated by periods of intense unhappiness. 'I'm sure I wouldn't have been happy if I'd been in the city either. It was the time of life not the place.' Does he ever feel guilty that an apparently idyllic childhood left him feeling lonely and sad? 'Yes. There is a deep knowledge now that one accepts a lot of nonsense one shouldn't and fails to appreciate a lot of the wonderful things one should - it happens in both directions.'

Fry went off to prep school as a boarder at seven, later moving to Uppingham. 'Norman Douglas, one of the writers I most admired when I was a teenager, had been a pupil at Uppingham and said the only decent thing was cheese from nearby Stilton and pies from nearby Melton Mowbray. And that's probably true.' Fry recalls 'standard public-school fare ... grey lamb, strange puddings', but nothing exceptionally awful either.

What does discomfort him a little now is the manner in which the boys treated the waiting staff. 'You were served at the table by local girls who worked in the kitchens. They were known as skivvies. It sounds awful, doesn't it? But I think the most awful thing, that would set people's teeth on edge today, was the casualness with which you accepted their place in your world. When your water jug was empty you'd just carry on talking and hold it up in the air. It'd be taken out of your hands, and you'd just carry on talking until it was eventually plonked in front of you full. You're not fundamentally evil, inconsiderate and unpleasant, you just do what is done, you know. Boys are fantastically conventional.'

Conventional? Not Fry. At 17, he was racing around London with a stolen credit card and living a life of fantasy. He did not, however, spend a lot of money in smart restaurants. 'I didn't have a particularly high taste in food - when I was staying in hotels, I probably had room service.' Instead, he was entranced by cocktails and, in particular, the bar at the Ritz. 'I loved the idea of cocktails. It was around the time of the film of The Great Gatsby. I had a suit with very baggy trousers and I'd go to the Ritz to have Brandy Alexanders.'

It couldn't last - and it didn't. He was arrested and sent to Pucklechurch prison, near Bristol, now known as Ashfield. 'I can't remember the food there - I'm sure it was ghastly. But I do remember trays. They had little compartments in them - the peas would go in one part, the mashed potato in another - and they used those strange ice-cream scoops, so it made an exact little dome. Mashed potato tastes so much less nice when it is in that shape for some reason.' Fry went on to Cambridge University, which is where he says he learned how to eat properly in restaurants.

These days most of his eating out is in London, where he has another home, and in one of three regular haunts: 'People in my world, it's unavoidable, we eat out at the Ivy, the Caprice and J Sheekey.' In fact, Fry has been going to the Ivy since before it opened: former owners Jeremy King and Christopher Corbin asked him to test out the restaurant before they let the public in.

'I went for a week while it was "previewing", as you might say, to come and test out the stations. I tested out different places and gave them notes. They'd ask about the lighting, the acoustics. And the reason they are such great restaurateurs is that they really understand how important things like the distance between tables is.'

Though King and Corbin have sold Fry's favourite three restaurants, he remains loyal. Other places 'are far more likely to be full of people who are not used to sitting around people they've seen on television, who are far more likely to come up and ask for your autograph on the programme of the musical they've just seen. And that's boring. But at Sheekey, the Caprice and the Ivy, you just know you'll get a table, and you know that you won't get ridiculously hassled by people. At the Ivy there's always someone more important than you, so no one's going to stare at you.'

Not that the people who work in showbusiness are themselves beyond gawping. 'I remember years ago being at Joe Allen's with Alan Bennett, Russell Harty and Alan Bates. I'd only just started out, I'd left university about a year before and I was like a pig in shit, I couldn't believe it. A lot of people were looking at us.

'But then while we were chatting away, in came Laurence Olivier and Dustin Hoffman. It was just hilarious: Alan Bennett was saying to Russell, "Go on, you're friends with Olivier, say hello." And Russell said: "No, everyone'll just say there's that Russell Harty sucking up to Larry Olivier ..."' A compromise was reached: Harty raised his glass in Olivier's direction. To the chat-show host's great relief, the actor reciprocated.

Our meal is drawing to a close. Fry scoops up the last mouthfuls of pork and raises his own glass. Lunch is over, and he leaves. I check the price of the wine with the waiter. I don't think The Observer's editor need worry himself.

· Stephen Fry stars in Kingdom on ITV1 next year