The Hogwarts Express does not depart from Platform 13 and a half to get to Warner Bros Studio Tour - The Making Of Harry Potter. You don't get on a train either, but a special, black Harry Potter liveried shuttle bus that sets off from Watford Junction at bus stop number 1. The 15-20-minute journey becomes exciting as soon as you spot the two huge studio hangars J and K, rising like a mirage amid the shimmering acres of the former Leavesen airfield.

Before we have even gone inside, my son John spots the white dragon emblazoned on the front of the building. "It's from Harry Potter and the Deathly Hallows - Part 2," he says knowledgably, "it's awesome. When it rose out of the chamber, its wings were so deadly it slew all these goblins and the ones that survived were burned down with its fire." I sense his excitement is suitably whetted.

Inside we collect our passports - there are six stamps to collect as you tour the attraction and 15 golden snitches to hunt.

As we wait in the holding room, we discover how 'the making' began back in 1997 when Nisha Parti from Heyday Films alerted her boss David Heyman about JK Rowling's first book and he brought it to Warner Bros, who optioned it before the year was out. Although only an introduction, this footage and the ensuing cinema presentation, prepare you for how big and impressive the tour actually is.

The Great Hall is a fitting starting point with robed, yet headless, characters inviting us to marvel at the bedecked dining room; at any moment you expect to be trampled by magical students arriving to be corralled by the sorting hat.

Blink in the next room and you'll miss something - a 360 degree panorama of Potterdom awaits from the Gryffindor dormitory to Dumbledore's office, Hagrid's Hut and beyond. You really need to pace yourself here as the layout meanders. There does not seem to be a map available either online or in the lavish souvenir guidebook (£9.95), so look closely for the signposts or hire a digital guide (£4.95).

At The Potion Room we gawp at more than 1,000 glass bottles, apparently filled in part with toy animals from London Zoo that have had their tails or legs cut off and bits stuck on. We peer into cases crammed with mechanical curios - the Philosopher's stone winks at us and it's impossible not to marvel at all the stuff that went into making these movies so spectacular and unique. While we might wonder how on earth you could possibly need 17,000 wands, the answer to this puzzle and numerous others are available if you ask.

The helpful and informative staff are happy to oblige if you have any questions or just want to pose for a photo opportunity. If you're wandering around gazing at the ceiling in pursuit of a snitch they'll give you a clue and will make sure you stamp your passport correctly for the best embossed effect.

Having seen the tour in its early stages, I was amazed at how much more detail has been added. In the broomstick room you can have a ball flying over scenes from the films and take a souvenir photo home with you (priced from £12). After this you can take a breather in the open air and snap a family portrait outside Privet Drive in the Ford Anglia, astride Hagrid's bike or beside the Hogwarts Bridge. The most impressive layouts come later in the tour - Diagon Alley is a sheer delight and the intricate model of Hogwarts Castle is brilliantly conceived and truly breathtaking. Move over Cinderella and Sleeping Beauty - there's a new palace in town.

Family ticket: £83, glass of Butterbeer: £2.95 wizarding wand: £24.95. Bargain buy - five Harry Potter lapel pins: £9.95.

Details: Attraction opens March 31, pre-booked tickets only from www.wbstudiotour.co.uk

The Harry Potter bus service leaves at 30-minute intervals throughout the day, £2 per person return: www.mullanysbuses.com