LOOKING back on it, the Weasley house would have been the best place to do it. Just a little name-tag, slipped discreetly into my pocket, a couple of buttons, or one of those scrunched up copies of the Daily Prophet. No one would notice -- not in the middle of all that junk, the dried-out bread, the scraps of paper, the bowls of trinkets.

'They're from the Weasley house, you know,' I would say to my ten-year-old nephew when I got back. 'There was a table in the corner with all these books on, and you should have seen the clock with the pointer for 'Tea-time' and 'You're late'.' But the sweat was crawling down the back of my neck, my heart was thumping faster than the Hogwarts Express train, and my fingers had seized into semi-paralysis. 'Move on, hurry up now,' one of our set escorts muttered. I've never been the world's greatest thief.

It was only later that I realised I was not alone. Who doesn't have a son or daughter or friend or nephew who would kill for a piece of Harry Potter's world? More than 40 million copies, after all, have been sold of Goblet of Fire and the first movie was, at time of opening, the biggest blockbuster of all time. A souvenir from the set would have a high currency in the playground back home. 'Look what I got,' said a fellow hack, pulling out a small dog-eared photocopy of a globe. 'That's nothing, what about this?' said another, slyly slipping a small book from his pocket, Magic for Muggles. 'It was in Flourish and Blotts. I got it when the lights went off.'

The Harry Potter set is a universe of its own. A huge 'world of plaster' packed into Leavesden studios, a one-time Rolls Royce factory, with its own fire engines, medical centre, a school for the child actors, toilets for 'children only' and, of course the specially constructed Hogwarts Great Hall, common room, infirmary, Chamber of Secrets, and Privet Drive. Inside its airy hangars are rows of marquees, vans housing dressing rooms, a fenced-off junkyard containing the giant chess pieces of Philosopher's, and here and there a piece of magic -- an animatronic mandrake that looks like a cross between a foetus and a rubber plant, a lonely blue Ford Anglia, a discarded strip of set leaning up against the wall, saying in bloody letters, 'The Chamber of Secrets has been opened. Enemies of the heir beware'. It's like Universal Studios without the rides, already well on its way to what, by rights, should be its final fate: the Harry Potter Theme Park.

Each day the set welcomes countless visitors: friends of the crew, classes of kids from cast members' schools, nosy, pilfering journalists and the occasional fevered fan. As Jason Isaacs, who plays Lucius Malfoy says, 'I have brought, frankly, a disproportionate number of children to the set given the size of my part. Chris [Columbus, the director] always puts the headphones on them and lets them say cut and action. And no one loses their temper. There seems to be very little pressure for time. I think that's probably because the whole film's budgeted around filming 13-year-olds.' I can't believe we journalists are alone. No one, I suspect, steps on set without some little voice niggling in their ear, 'Bring us something back. Go on. Please. Oh please.'

The day begins early. There are ten of us, a motley crew of Muggles from all over Europe, setting off in a bus bound for Leavesden. I half expect us to be blindfolded, searched and then driven to a secret location. But this chamber of secrets doesn't seem so secret: there are no rottweilers, no searchlights, no armed checkpoints, just a security officer and a couple of slobbery 12-stone mastiffs who play Hagrid's dog Fang. As we drive through the entrance, there aren't even any crowds of fans, no paparazzi photographers, none of that Pottermania.

As Columbus, tells us later, 'Look where we are out here. You couldn't find us if you knew where we were. It's amazing we don't have anyone waiting outside the gates. There aren't crazed fans. It's an odd situation where you have the second part of a blockbuster film and no one is here. No one comes around. When I first moved to England I thought, 'Oh great, the kids are going to be plastered all over the first page of The Sun'. But it didn't happen. The reporters here have given them their privacy. They seem to be happy with Jordan and all that other stuff.'

THIS is Columbus's second and last, for the moment, Harry Potter film. Chamber of Secrets, based on the second novel by JK Rowling, finds Harry and friends Ron and Hermione battling against a dark force which is terrorising Hogwarts school. During their quest to discover what is behind this evil presence, Ron and Harry end up flying in a Ford Anglia almost colliding with Hogwarts Express. The film also features new characters including Gilderoy Lockhart, professor of defence against the black arts, played by Kenneth Branagh. Columbus hands over the next instalment, The Prisoner of Azkhaban, to Mexican film director Alfonso Cuaron. But it's not that he hasn't enjoyed the experience: 'I just haven't seen my own children for dinner for two years and I didn't want to miss them growing up.'

Our first stop is Diagon Alley, a Pottermaniac's treasure trove. There we find Flourish and Blott's piled high with 17,000 books, some swirling in gravity-defying curves up to the ceiling, copies of Wand Welfare, Celestial Studies and Elf and Safety, the Quidditch shop with it's mace-like bats and the odd Nimbus 2000, and out on the cobbled street, bat spleens made of grass and dragons livers from mud and old roots. Next, past the 250ft screen painting of the Scottish hills to the Great Hall, empty now, apart from a few long wood tables and an elaborately designed house scorer. Its gimmicks are all theatrical, not magical -- secret doors in the walls of the set for crew to get in and out, cushioned pads on the steps to cushion the fall of the lead actors. Then, in a whirlwind, on to the infirmary. There, I press my nose up against a medicine cabinet and strain to make out the labels on the glass bottles inside: Gold of U, Beef Tallow, Rose Lozenges, Wind Pills. Another journalist slips up behind me: 'I think this is Daniel Radcliffe's personal stash.'

The place is like a ghost town (only really alive, I suspect, when the director shouts 'action' or the computer animators weave their digital magic) so it's a relief when we finally meet the actors. Daniel Radcliffe walks in, dressed as though he's just come out of the classroom. He has just finished a scene in which Harry and Ron use the Polyjuice potion to turn them into Crabbe and Goyle. 'Great scene,' he explains, 'I've got this arm and there was all these things pumping it, so my hand was bubbling. It was really cool, gross but cool.' He is taller and thinner in the face than he was a year ago. His voice has broken and he says 'cool' a lot, and he is into, of all things, Seventies punk. 'Old punk rock,' he says, 'like The Sex Pistols and Iggy Pop and The Stranglers, The Undertones, The Clash. Is that okay to say that? It's not just a phase.' His life has changed, undoubtedly, since he became the real Harry Potter. For one thing, with his personal tutoring at Leavesden, he's getting the best school grades he's ever had. 'There are differences,' he said in his very polite, hesitant way, 'between before and after Harry Potter -- basically before I didn't have my face printed on buses. Now I get recognised, but I like being recognised because I'm an attention seeker. But I see my friends all the time and they come round and play PlayStation games and stuff, so I have a pretty normal life.'

One thing you immediately notice, is that Harry Potter isn't like most sets. There's more of a sense of permanency here, rather like being caught on a long-running soap or trapped in a timeless public school. No one really wants to talk too far into the future, but it looms -- an unknown territory, guarded in the mind of JK Rowling. Are they worried about the movies catching up the books? The fifth book, after all, is yet to be published. 'I wish people would get off her back,' sighs producer David Heyman, 'because the fact is that this woman delivered four books in four years and they were wonderful. Now she's taking the time to write the book that she wants to write and it will be fantastic. I just think people need to give her a break.' Will Daniel Radcliffe still be playing Potter by the seventh film, or will he have jumped ship to play Spider-Man 4, take up a career as a director or any of his other ambitions? 'I just want to concentrate on the second for now,' says Radcliffe wisely.

Meanwhile there's a durability to the set. Everything, of course, is made of plaster -- more plaster, they say, than was used on Gladiator -- but built to last, and with the kind of attention to detail not often found even in the real world. Theoretically, Chamber of Secrets, reusing as it does many of the sets from the first film, ought to be cheaper, but it isn't. The world has just expanded, 20 new sets have taken up a huge hunk of budget, and the crew have become even more anorakish and finicky in their obsessiveness.

'With a film like this,' says the deputy production designer, 'it's better for us to cover all bases, because, who knows what a kid's going to do in the shop. You suddenly think, 'Shit, why didn't we paint that? Why didn't we at least stain it with some teabags?' Look a bit closer, though, and the forgery starts to give way. The bread is hard, the vegetables are rotten, polystyrene cups of cold, whiskery coffee lurk in shadowy corners. Still, even in its lumpen fakery, it's a little bit magical. After all, nothing is what it seems. Open Gilderoy Lockhart's Magical Me and you find inside Alan Clarke's The Tories, take a wind pill and it's a smartie. Run your fingers through a cobweb, and, instead of finding a spider running up your arm, your hand is coated in a film of dried glue -- fired, it turns out, through a compressed air gun.

The whole place looks as though it's been made by a bunch of JK Rowling anoraks. What's amazing is, apart from the few location shoots -- Alnwick castle as Hogwarts, Durham cathedral's cloisters as the seventh floor corridor, Christchurch college Oxford as the entrance hall -- it's all been created from scratch. The detail is so intricate: the back covers of books, the tiny designs delicately carved onto the Hogwarts house scoreboard which will almost certainly never be seen on film.

There is, it seems, another parallel universe that exists here, a working one, of children, actors, parents, visitors, crew -- and it leaves its sign all over Hogwarts. In the end, that's what overwhelms the visitor. Inside one leather-bound schoolbook, I came across the doodles and scribblings of extras, bored no doubt, as they sat through take after take of the Lockhart's test scene. 'I've got a feeling we're going to be here for some time,' said one. 'I feel like poo,' another. 'This sux, big time.' 'Kiss, kiss.' 'Twinkle, twinkle, little star. I love u for what you are.'

It's not difficult to imagine that the set has both its more banal and comic moments. Radcliffe, it tuns out, has a reputation as a terrible corpser and practical joker. Just the twinkle in someone's eye can set him off. On the last film, he confesses, that he reset the language on Robbie Coltrane's mobile to Turkish. 'I haven't tried that again,' he says. 'I don't think I would still be here.' Emma Watson, who plays Hermione, talks of note-passing and gossip. 'Not much romance between the main leading parts, but there has been some romance around the set. I couldn't give away any names. As you can imagine after a week of working in a boiling hot great hall, where the food stank, everyone was just bored to death. It can get fun, because Dan got everyone to laugh by standing on the table and dancing with Robbie Coltrane. That is a very vivid memory, the macarena, the can-can, everything.'

Isaacs tells a tale from the Quidditch shoot. 'Chris came up to this group of extras, and he's saying, 'Get a bit more involved. Just remember he's been hit by the bludger and he's going for the seeker.' One of the wizards turned to another and said, 'What the f**k is a seeker and what am I meant to think about it?' Then Alan Rickman said, 'It depends what house you're in'.'

Ignorance like this, however, is rare. Rowling's books seem to bring about the trainspotter in everyone. No one seems just content to play a role. They all want to understand the world, know their characters inside out, become an essential cog in the fantasy. Isaacs, for instance, insisted upon a decadent look for his sneering Lucius Malfoy -- cape, fur, silver-topped walking stick and flowing blond hair, rather than the pin-striped suits the designer originally suggested. 'I said well, he's an aristocrat Lucius Malfoy, he comes from countless generations of would-be wizards and he's like those hideous members of the Tory party here who might be in government but really rule the country behind closed doors and their sofas are tatty and their jackets are tatty but their grandfather wore them. There's no point in being in a film about wizards and dressing like a businessman, frankly. Might as well dress like a wizard.'

It's this that, for all its other faults, made The Philosopher's Stone such a successful film -- this dedication to Rowling's creation. It's this also that makes me all the more determined to bring back some souvenir. A trophy from the studios is almost like a trophy from the magical Hogwarts itself, the closest, at least, you can get. 'Please,' a little voice beseeches in my ear, and I imagine trying to explain that I've asked the PRs and they haven't got anything to give.

Standing in the Chamber of Secrets, 250ft long, a great lofty cavern with towering snakeshead carving, I scan its placid black waters. 'Be a great place to have a wrap party,' says one of my fellow journalists. But all I can think about is that this is the last set we're visiting and there's not a loose piece of plaster I can pinch. Then, I see it, there on the floor. A grubby Mandrake leaf. The full-sized plants, no doubt, will be coming to a store near you as merchandise this Christmas, but it's not the same. This is magic. This is the real thing.