JO KESSEL and her family are bewitched by a tour that lifts the lid on the world's most successful film franchise

LET ME explain the rules of Quidditch," says my nine-year-old son Gabriel. "It's a sport where you've got to catch the golden snitch," interrupts his twin sister Nathalie. "In the Philosopher's Stone," adds Hannah, six.

We're heading up the M1 from London towards Watford, en route to The Making of Harry Potter Studio Tour, Warner Bros's latest attraction which opened yesterday.

My husband's never read a JK Rowling book let alone seen a film and I'm only marginally more clued-up, which is why our Harry-obsessive youngsters are filling us in. Gabriel sighs, "but most people already know this."

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The new walking tour is based at Leavesden Studios where the eight Harry Potter films, at more than £5 billion the highest-grossing film franchise ever, were shot.

It offers a behind-the-scenes peek at some of the films' most iconic sets, props and costumes, as well as hiding 15 golden snitches (winged balls the size of oranges) for visitors to try to spot.

Entering the foyer my three stare wide-eyed at Weasley's flying car, a pale blue Ford Anglia, suspended from the ceiling. They then place their hands in moulds of Harry and Hermione's palm prints. "Ooh, I've just touched Daniel Radcliffe," gushes Nathalie.

The tour begins with a brief introductory film featuring Radcliffe and co-stars Emma Watson and Rupert Grint before the screen dramatically rises to reveal the inside of the Great Hall of Hogwarts, the school for witchcraft and wizardry.

It is long, cavernous and Gothic, and so real that we half expect lights to dim and the director to call: "Action". Here are the very same banqueting tables where 400 Hogwarts pupils feasted and etched graffiti. The detail and craftsmanship are phenomenal.

Beyond is the boys' dormitory and the Gryffindor common room, where half-eaten boxes of Bertie Bott's Every Flavour Beans lay strewn on tables.

There's Dumbledore's office, Hagrid's hut and the Weasley kitchen. Here we get to operate an animatronics self-washing frying pan which, frankly, should be an obligatory device in all households.

The exhibition is full of excited chatter, with youngsters spouting facts to ignorant parents (we're clearly not alone) and jumping for joy on spotting hidden snitches (we sight a feeble five out of 15).

My favourite set is the Potions Classroom where pickled animals float in bell jars. The children love Diagon Alley, the narrow boutique-lined street where wizardry paraphernalia such as wands and broomsticks is sold.

Talking of broomsticks, my lot take turns donning cloaks and straddling a Nimbus 2000 (a premium model). Courtesy of special effects (a green screen and wind machine) they can watch themselves in action on a screen as they soar above the streets of London and across countryside. Photos are available and memento DVDs are planned.

Flying is thirsty work so we move on to the "backlot" outside, home to exterior sets for the films such as Privet Drive and Hogwarts' bridge. Here, there's a refreshment stall selling Butterbeer, the infamous Hogwarts beverage. At £3 a glass it's not cheap and doesn't look particularly appetising: a pale yellow concoction topped with thick froth. Its creamy toffee flavour is unexpectedly tasty, however.

The tour's pi¨ce de r©sistance is a giant 1:24 scale model replica of Hogwarts. Used for filming exteriors of the wizard school, its sheer detail pays tribute to the British film industry.

Goods are pricey in the adjacent gift shop: a broomstick for £25, a notebook £15 and a replica Professor Dumbledore gown for an eye-watering £445.95.

We content ourselves with some Every Flavour Beans, small jelly sweets whose flavour menu is an A-Z of repulsion: bogey, earwax, earthworm, rotten egg or vomit.

Tentative tasting of these on our journey home turns into greedy consumption. They're spicy, tangy, sugary delights and my husband is hooked. Not just on Every Flavour Beans, but on Harry Potter full-stop.

In fact, he's become a bit of an obsessive, vowing to keep coming back until we've spotted all those golden snitches.

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GETTING THERE

Warner Bros. Studio Tour London - The Making Of Harry Potter (0845 084 0900/ wbstudio tour.co.uk).

Admission: £28 adult, £21 child, £83 for a family ticket (2 adults 2 children). All tickets must be booked in advance.