Charlie and the Chocolate Factory, Theatre Royal Drury Lane

It’s all stick and no lollipop, a chocolate box stuffed with nothing but empty wrappers: what a walloping letdown this intensely anticipated musical based on Roald Dahl’s perennially popular 1964 children’s book turns out to be.

With songs by Scott Wittman and Marc Shaiman – the team behind the irresistible feelgood hit Hairspray – a book by the highly respected playwright David Greig, and direction by the Donmar Warehouse founder and Oscar-winner Sam Mendes, it ought to be a giant peach. Instead, it’s as bland and sugary as cheap confectionery. And with so little to savour of Dahl’s delicious treacly darkness, it’s effortlessly kicked into orbit by Matilda and her sassy cohorts, singing and dancing up a storm of wit and wonder over at the West End's Cambridge Theatre and on Broadway.

Hodge's Wonka comes across as little more than avuncular and faintly world-weary

For a show that purports to be so starry-eyed over the potent magic of imagination, it’s clod-hoppingly literal – and nowhere is this a more serious problem than in the dismayingly dull first act. It begins with a needless cartoon by Dahl illustrator Quentin Blake, in which we are tediously introduced to the chocolate-production process. Things hardly get any more exciting when we meet our pint-sized hero, Charlie Bucket (played at the performance I saw by Jack Costello, pictured below right) picking his way through a giant rubbish heap, courtesy of designer Mark Thompson. “How do you do?” he enquires of the dubious treasures he unearths – a single glove, a broken umbrella – before trotting off to the dismal home he shares with his cheerful but impoverished parents and his four winsomely eccentric, bed-ridden grandparents. It’s tooth-rottingly sentimental.

A welcome dash of colour is delivered when Dad wheels in the erratic TV so that Charlie can follow the worldwide search for lucky Golden Ticket winners – the select few who will get the chance to enter the hallowed portal of the Willy Wonka Chocolate Factory and meet the mysterious master chocolatier himself. Appearing on the over-sized set are that victim of the childhood obesity epidemic, Augustus Gloop, a lederhosen-sporting Bavarian boy and his fond Mutti, singing along with an oompah band; Veruca Salt, a blonde mini-monster in pink tutu and tights, with her long-suffering, over-indulgent tycoon father; Violet Beauregarde the chewing-gum addict, transformed here into a ghetto-fabulous, blinged-up rapper with a fame-hungry soulman dad (Paul J Medford); and, most repellant of all, Mike Teavee, a pathologically aggressive video-gaming obsessive whose pill-popping mother is, inexplicably, a beehived Sixties housewife.

It’s not until the act one closing number, though, that we finally get our first proper glimpse of Douglas Hodge’s Willy Wonka – and even then, we don’t actually make it into the factory itself before the interval. Moustachioed and dapper in his bright top hat and tails, Hodge cuts a genial but curiously unengaging figure. Dahl’s Wonka may be enigmatic, but a dramatic portrayal demands some sense of character; Hodge comes across as little more than avuncular and faintly world-weary. It’s not a lot to suck on to see us through until the second half.

Happily, though, there are a few treats to come as Charlie and his doughty Grandpa Joe (Nigel Planer) potter through Wonka’s secret sweetie-land. Chief among these is Jamie Harrison’s puppetry, which staffs the factory with adult, full-sized Oompa-Loompas with little puppet legs. Their first entrance, singing and dancing in an eye-popping, high-kicking row poised on an industrial pipe, is the show’s highlight; also hugely appealing is the shrinking of the loathsome Mike Teavee to a tiny, shrieking action-figure in Wonka’s experimental teleporter, and the Nut Room, where a slightly menacing horde of bushy-tailed squirrels sorts out the wholesome kernels from the rotten, and at whose furry paws brattish Veruca meets her grisly end.

Yet still there are further disappointments. Thompson’s Chocolate Room, with its minty grass, candy blossoms and chocolate waterfall rippled like brunette mermaid hair, is luridly bright yet strangely flat; Jon Driscoll’s pedestrian video projections are singularly unenchanting. Wittman and Shaiman’s numbers roll flavourlessly by until they merge into a single slick butterball of inconsequentiality; and even the choreography by Peter Darling – who can usually be relied upon for terpsichorean dazzle – lacks thrills and the lift-off of sheer exuberance.

The climactic flight of the Great Glass Elevator – which, oddly, is executed to the strains of "Pure Imagination" from the 1971 film version, shoehorned in here for no very obvious reason – may well be jaw-dropping; it was malfunctioning when I attended, so I can’t judge. But even if it executed quadruple loop-the-loops over the heads of the audience, it’s unlikely that it could have fully compensated for so much that is lacklustre in what precedes it. Perhaps affection for Dahl’s story and awe at what spectacle the show offers will be enough to ensure steady business at the box office. But the sad truth is, this ain’t no golden ticket.

• Charlie and the Chocolate Factory is booking at the Theatre Royal Drury Lane till 31 May 2014