Not even in his cruellest moments could He-Who-Must-Not-Be-Named have conjured up a spot quite as bleak as an industrial estate outside Watford on a grey March morning. Look past the vast, expectant car park and ignore the chugging of last-minute trucks, however, and something distinctly unmuggleish is going on deep within the huge, mustard-coloured hangar.
Despite its location — not to mention the jarring corporate-speak of its official title, Warner Bros Studio Tour London: the making of Harry Potter — the exhibition promises to bring true wizard fans closer to the heroes and villains than ever before. From the end of the month, they will be able to stroll down Diagon Alley, peer into Professor Snape's potions class and gaze around Dumbledore's office as the set where the eight films were shot opens to the public for the first time.
To commune with Harry, though, parents will first have to run the gauntlet of the lobby, with its cafeteria, Starbucks, gift shop and – a thoughtful touch, this – cash machine.
After a quick, teasing film in which the series' young stars hint that the secrets of the films will be laid bare – "You'll never look at Quidditch the same way again", promises a cryptic Daniel Radcliffe – the screen shoots up to disclose a pair of enormous wooden doors. "Yes, ladies and gentlemen," imparts one of the legions of enthusiastic and spookily Potter-literate guides, "this is the one and only Great Hall of Hogwarts."
And so it is. Although the flagstone-paved floor, battered oak tables and smoke-blackened walls are instantly familiar from the films, it is a shame to glance up at the "enchanted ceiling" and find a black metal gantry and harsh spotlights rather than ancient beams and the odd reticent owl. From that moment on, the veil is well and truly lifted; the stone gates of Hogwarts, imposing as they are, ring hollow when knocked and the lycanthropically inclined Remus Lupin is revealed to be a beautiful piece of animatronics rather than a supernatural fiend.
But, the more technical trickery reveals its sleight of hand, the more the sheer ingenuity and effort that went into the films becomes apparent, from the 350 handpainted portraits to the vast, 1:24 scale model of Hogwarts which took 86 people six months to build. Were a single person to have laboured over its plaster and MDF walls, buttresses and towers, it would have taken them 74 long and lonely years.
Equally Herculean is the marketing push that has gone into the cavernous gift shop that lurks like a ghastly, wallet-sucking Dementor at the end of the tour, flogging baseball caps, scarves, umbrellas and motion-activated Gryffindor swords. A replica of Dumbledore's robes can be snapped up for just 495 of your muggle pounds, while a Nimbus 2001 broom will set you back £199.95. Wands are more competitively priced at £24.95.
The finishing touches are still being put to the £100m attraction – hence the builders singing along to Careless Whisper as they finished off the roof on 4 Privet Drive – but all the signs suggest it will enchant visitors as much as the books and films that gave rise to it. Jan Bantic, a 13-year-old from the Isle of Dogs who described himself, a little redundantly perhaps, as "a huge Harry Potter fan", paused to give his thoughts as he ambled along Diagon Alley. "It's really, really amazing being here and seeing the amount of effort that went into it that you don't really see," he said. "I don't know how else to describe it."
Was he not worried, though, that the visit would break the spell? He shook his head. "It just makes you appreciate it a lot more. There's even more magic because you can see what went in how they created what went on. I'd say the magic is still very much intact."
• Warner Bros Studio Tour London: The Making of Harry Potter opens on 31 March
