First night: Sam Mendes' Charlie and the Chocolate Factory is a skilful confection, but leaves you wanting more

Who, in their right mind, would choose to be an Oompa-Loompa - part of what is effectively a sweet-toothed slave-labour force? Still, they are full of vim, ingeniously created (a hectic hybrid of human and puppet), and, in one number, they roller-skate about like crazy and thus can be included in the more successful aspects of this much-anticipated stage musical version of Charlie and the Chocolate Factory.

Sam Mendes' big-budget production has opened after a whole month of previews and in the daunting wake of Matilda, that other Dahl-derived musical that is still riding high in the West End and on Broadway. Comparisons, invidious and otherwise, are inevitable.

In some ways, you could argue that the makers of Matilda had it easier. The story of the pint-sized prodigy's battle with her repellent parents and sadistic headmistress has the kind of dynamic emotional conflict that people of all ages can identify with.

By contrast, Charlie begins and ends as the soul of unselfish sweetness and light. His family are so docile in their destitution that they make the Cratchits of A Christmas Carol look like hardened Communists. And there's never any doubt that Charlie will pass Willy Wonka's test or that the Golden Ticket-holding brats will eliminate themselves through their greed and sense of entitlement.

This musical version is certainly blessed in its leading performers. As the enigmatic chocolatier, the brilliant Douglas Hodge is more school of Gene Wilder (from the 1971 film) than of Johnny Depp who seemed to be channelling Michael Jackson, Marilyn Monroe and Anna Wintour in his freaky portrayal for Tim Burton in 2005.

Leaving everyone guessing about his sanity, Hodge's Wonka is a wonderfully unstable cross between a visionary with a screw loose and a Prospero-like figure with a serious game-plan and a yearning to retire. He delivers a master-class in comic timing as he tosses off complicated patter-songs with a quiet, almost insolently negligent ease and takes the little horrors on a continually wrong-footing tour of his factory's wacky marvels.

These are evoked here via the technical wizardry of whooshing computer graphics, hydraulic lifts and a Mark Thompson set on which the chocolate waterfall resembles a heavily sweating Curly Wurly bar. You won't be disappointed with how they achieve the grisly come-uppance of the poisonous poppets.

It's in the nature of the story, though, that Wonka makes a dramatically delayed entry. This means that, for the first half of Mendes' production, we are largely marooned in the Dickensian shack of the Bucket family with its quartet of bed-ridden Beckettian grandparents.

At the performance I saw, Jack Costello's adorable portrayal of Charlie suffused the proceedings with a lovely sense of the boy's pining purity. But the best bits of this rather static stretch are our initial glimpses of the brats.

Wittily updated, they appear in a giant TV that's like a stage-within-a-stage. The bubble-gum-chewing Violet Beauregarde has been reconceived as a Britain's Got Talent-style rapper replete with grovelling entourage; Mike Teavee is a computer-games maniac etc.

The score by the Hairspray combo of Scott Wittman and Marc Shaiman is tuneful and wholly unmemorable (the only song you come out humming is “Pure Imagination” the captivating Newley/Bricusse number borrowed from the 1971 film). But it rises to a nicely mischievous wit with the yodelling Germanic Gloop family and with Iris Roberts's hilarious Mrs Teavee, a delectable throwback to the suburban 1950s, who sings that “Medication set us free/One pill for Mike and two for me”.

Peter Darling's choreography offers an attractive mix of East End knees-up and synchronised Oompa-Loompa antics but the glorious finale with Charlie now kitted out as a mini-Wonka who mirrors his mentor's zazzy, off-centre motions makes you wish there were more of it. Very engaging but rarely elating, this show is a skillful confection that doesn't quite produce the inspired sugar-rush of magic that's required.

Booking to May 31 2014; 0844 871 8810