How best, then, to explain Heat to those unfortunate enough never to have come across it? Imagine a magazine version of a retirement home for Big Brother contestants. The nation peers through the windows at the contestants' final gasp for attention before the death rattle of their celebrity rasps out. "My God," says everyone outside the window. "Did you see that?" Then they wander away from the littered corpses to buy mince for tea while a new batch are hurriedly stretchered in.

"There's no coincidence that the rise of Big Brother and the rise of Heat were of a similar scale and pace," says 38-year-old Frith. The programme meant a new celebrity born every five minutes. "You could pick and choose, and that's one thing that made the job so great. Britain and the tabloid culture just creates celebrities all the time." It's wot put the Great in Great Britain, innit? Jade Goody. Ziggy. Or was it Zoggy? Last seen setting out for Hollywood in a vain bid to avoid the retirement home.

Big Brother was certainly crucial in contributing to that triumphant triumvirate of covers that Heat rotated ad infinitum. Posh Spice, Jordan, Big Brother. Posh Spice, Jordan, Big Brother. Posh Sp… you get the picture. Usually a selection of people who wore sunglasses indoors in the hope of convincing you that you knew them well enough to necessitate their hiding.

The 'celebrity bible', as it became known, Heat was Frith's baby. It was so very nearly stillborn. When it launched in 1999, Frith was deputy editor, but within just a few weeks the sales figures were so dismal it was threatened with immediate closure. Frith took over and proved himself a remarkable editor, taking sales from 38,000 a week at their lowest to a peak of more than 500,000. He won every major publishing award, including the PPA editor of the year and the Mark Boxer Award for Outstanding Achievement to British Magazines, and went on to present BBC's Liquid News for a year. But this year, Frith left Heat to write his first book, The Celeb Diaries, a magazine version of Piers Morgan's expose of newspapers, The Insider. The man who helped fuel Britain's decade of fame-obsession claims he left because the celebrity world was no longer fun. It had all become, simply "too dark"…

FRITH CAME FROM Sheffield, not exactly a city with a reputation for glamour, but this was the 1980s and it was enjoying a surge of musical talent. "A lot of the big music of the day – Human League, ABC, Heaven 17 – came out of there, so I used to see the people from those bands around Sheffield all the time." His father was in a local band called the Cherokees, and Frith was obsessed by music and music magazines. "They kind of spoke my language. And magazines are, at their heart, for enthusiasts. I loved how infectious their enthusiasm was and the way they loved the world that I loved."

After graduating from college in London, Frith spent a short time at Smash Hits before heading on to Heat. But seeing those Sheffield bands at close quarters, sniffing fame close up, had been just as formative as the music. "What you have to understand is that I am an ardent people-watcher. I am the kind of person who friends come to for relationship advice. I am just intrigued and fascinated by how people get on or don't get on."

He's very personable, Frith. Tall, a gangling 6ft 5in, and generally regarded as 'a nice bloke' rather than the stereotypically ruthless showbusiness editor you might expect. He gives the impression that while he wants celebrity exposés, he doesn't really want the celebrities to hate him for it. Tricky.

As someone whose job often involves interviewing celebrities, I read his book with interest. It's gossipy, funny and an entertaining read. But it's also stuffed full of contradictions, which Frith shows a celebrity-like ability to sidestep.

His editorship of Heat marked a time of intense celebrity interest in society as a whole. But it also marked a period of breast-beating about the demarcation lines between privacy and public interest. As Heat battled with magazines like OK! and Now for the celebrity market, the journalists fighting to get the story over their rivals didn't always follow the rule book. Hilariously, Frith gets aerated when celebrities use the same tactics back to him, and seems oblivious to that irony.

He gets annoyed, for example, when Jude Law's lawyer gets his private number. And take his run-in with DJ Chris Moyles. Heat published a series of photographs purporting to show Moyles picking his nose. On his Radio 1 show, Moyles said he had rubbed his nose, and started berating Frith. The station was being listened to in the Heat office, and the place drew to a silent halt. Embarrassed, Frith phoned up Moyles's production team and said to tell Moyles to stop right now. Instead, Moyles told the listeners about the call. "In that instant, I am torn because I know it is really good publicity, but when it's being done publicly and broadcast to a group of people who are meant to respect you as the boss, it's quite hard going. I'm not one of those editors who comes in shouting and screaming every day to indicate my authority. I do it in more subtle ways. I try to be as nice a guy as I can, but because of that you have to show you are boss in lots of different ways. You have to be right as much as possible… make good judgements… so to be undermined in a public way on the office stereo is hard to take."

Isn't he missing the point? He publicly humiliated Moyles but gets angry when Moyles does the same back. It's the same when Victoria Wood phones a member of his team for a documentary but pretends to be someone else. When I ask about it, Frith says, "My issue was that we are governed by strong rules of the Press Complaints Commission, and you are not allowed to ring up people under false pretences, so I felt her phoning up a quite junior member of the team was completely out of order." But wait a minute. He opens The Celeb Diaries with an account of a reporter called Dan Fulvio lying (to someone junior) in an attempt to get information about Cheryl Tweedy and Ashley Cole's wedding. If it had worked – which it didn't – Frith would have used the story, wouldn't he? "Fair point," he says mildly.

Did he, perhaps, struggle sometimes to justify what he did? "Morally, there's a question about a lot of the content all the time," he admits. "Moral dilemmas are called dilemmas for a reason. When things happened to make us really question things, we really did. It's very difficult. I used to say to the team, 'They don't teach you this at editor school…' It became my catchphrase."

He uses the magazine's fight against size-zero models as an example of its moral probity. Heat ran pictures on the cover of stick-thin celebrities and opened up a nationwide debate, with heavyweight papers like the Guardian taking up the issue and running with it. Their stance would, indeed, be quite impressive if we could completely believe it was the moral crusade Frith tells us it was. Except that in the preceding months, he had written in his diary about one of his rival magazines running a cover of celebrities looking awful and achieving massive sales. It turned around Frith's number-one rule that the cover shot had to be flattering. He needed a bit of that action.

And then there is the fact that Frith admits he didn't want to run the size-zero cover in the first place. (Not that much of a crusade then.) He was persuaded because he had nothing else. It achieved huge sales. "It was inspired by paparazzi pictures of celebrities looking skeletal," he says now. "It was the view of the team that sudden weight-loss is just not healthy. It was not something to aspire to, not attractive." And saying so was ever so lucrative.

But that was Frith's job, to tune into what people thought and wanted. It was the fact that he kept his finger on the pulse of the nation's interests that made him successful. When film star Ewan McGregor said Heat was "a dirty piece of shit", Frith retorted that he didn't edit it for McGregor – he edited it for 500,000 readers.

But inevitably there were times his judgement was awry. In December, the magazine produced a series of humorous stickers. One was of Jordan's son Harvey with the words, "Harvey wants to eat me." Now Jordan's son suffers from a medical condition that affects his appetite. The sticker was interpreted as making fun of a disabled child, and the fallout went on for weeks. Frith was seriously under fire. "It was very important to me to describe in the book the context in which we decided to do that sticker," explains Frith. "Jordan was on the cover every few weeks and she talked in a lighthearted, jokey manner about his extensive eating habits, and the sticker was a reference to that. I didn't appreciate people telling me it was malicious when it certainly was not."

Jordan began legal proceedings but halted them after a printed apology. Her anger was qualified: at the height of the row, her PR team was discussing featuring her products in the Christmas edition of Heat. Still, Frith admits his error. "I'm not one of those people who says grandly I have no regrets. It was a huge mistake. In the cold light of day, it offended people. We were wrong and we apologised." Of course, if you can't stand the heat, get out of the kitchen. A few months later, Frith did just that.

IN JANUARY, Frith came back after Christmas to depressing stories. There were pictures of Britney Spears refusing to hand over her children to her ex-husband and being stretchered out of her house. And then there was children's TV presenter Mark Speight being arrested and questioned after the death of his girlfriend. Frith was depressed. "I want out," he wrote in his diary. So what had changed? "Trust me when I say this," he says. "We had to deal on a week-by-week basis with the stories, and it was very hard to find lighthearted stuff for the magazine. I'm not the only one saying this. It's a very noticeable trend."

There is something slightly odd about this argument. The celebrity world has always had its dark side. Marilyn Monroe. Judy Garland. Jimi Hendrix. Jim Morrison. Karen Carpenter… the list is endless. The truth is that fame doesn't make you happy – it might make you very unhappy – and it certainly didn't take Spears to teach us that. Is Frith saying things are suddenly different? No, he replies, but he clearly interprets the question as a suggestion that the celebrity press has exacerbated the problems of fame. "Amy Winehouse and Britney Spears would have been troubled if they had worked down the local Superdrug. I don't think the things they are going through are related to press attention. I just don't believe that."

Bad example, I'd have thought. It's true that troubled stars are often just troubled people. But fame sometimes splits the psyche open, and Spears, who became famous at 15, is as good an example of the corrosive pressures of fame in the modern world as you're likely to find. She became part of a well-established tradition of troubled child stars who don't make it unscathed to adulthood.

So, if things aren't so very different, why leave? Frith says he worked furiously during the week but could always switch off at weekends – it didn't affect his private existence. But did he, perhaps, recognise that Heat was of its time and that he couldn't maintain the amazing circulation he had built? "I had been a decade at the magazine and I felt that was enough for anyone. Could I have taken sales any higher? I'm not sure I could have done, so that seemed a good point to leave." He questioned in his diary if the celebrity obsession could continue, but now – perhaps out of loyalty to his old title – backtracks furiously and insists it's as strong as ever.

Gordon Brown said publicly that Britain's celebrity obsession is over. He wishes. Ironically, it's perhaps Brown who proves the reverse. Politicians are fond of saying it's substance that matters over image, but with Brown, clearly not. As chancellor, he became known internationally as the Western politician who had done most, within the confines of capitalism, to try to alleviate world poverty. His moral credentials – his 'substance' – should be obvious. But Brown is dour. He's not charming and extrovert. And you certainly couldn't imagine him appearing in Heat, stripped to the waist in Torso of the Week, as Tony Blair did.

So perhaps Frith is right. Britain's celebrity obsession lives on. Heat lives on. And what next for Frith? He and his girlfriend have a two-year-old son and are expecting another baby in the new year, so for the moment he's happy with the time off. "Without going into specifics, the advance for the book was good so I'm fine for a while."

Editors – like Frith and GQ's Dylan Jones – have almost become celebrities themselves, but it hasn't put Frith off going back to magazines one day. Celebrities spring from the unlikeliest sources in the new order. And at least this conversation we've had about his life and motives, he says irreverently, has saved him money on one of those celebrity necessities. The shrink fees.

The Celeb Diaries (£14.99, Ebury Press), by Mark Frith, is out now