In the middle of Roald Dahl's yukky kids' book The Twits is lodged the information that the eponymous terrible twosome had worked in a circus as monkey trainers. Irish playwright Enda Walsh has chewed on this nugget and produced a crushingly unfunny vaudeville that is nothing like the essence of Dahl and nothing much like the authentic Enda Walsh, either.

One can see the attraction of doing a Dahl show devoid of the soothing musical theatre charms of Charlie and the Chocolate Factory or Matilda, although neither of those blockbusters has anything as tasty as the too brief blast of "In the Bleak Midwinter" re-mixed with Slade's seasonal anthem when John Tiffany's production turns unaccountably into a Christmas show.

Chloe Lamford's design is a circular upturned drum, with a family of caged Welsh monkeys on one side and a makeshift caravan – bearing the itinerant circus trio of Yorkshire Terrier Man (Sam Cox), a Tattooed Fortune Teller Lady (Christine Entwisle) and a Handsome Waltzer Boy (Dwane Walcott) – lodged among the dismal trees.

As in Dahl's story, the play's all about reclaiming liberty from the dastardly Twits, who are played almost genially by Jason Watkins (in a humongous ginger wig and hair piece) and Monica Dolan (gimlet-eyed, Ken Dodd gnashers), as if channelling Noah and his wife in a mystery play.

The mystery in this play is, really, why did anyone bother? The monkeys dream of a new day with tea and cakes and Welsh hymns, while the circus mob, fleeing their wretched existence under a flyover in Leeds, itch to get out among the good people of Great Missenden (we are supposedly somewhere in Bucks) and purvey the wonders of their waltzer wares and doggie acrobatics.

The cruelty to animals theme of birds nailed and glued to trees, caged simians and performing dogs is extended into the retrospective crime of a murdered puppy, Baxter, which carries no more dramatic clout than any of the other diversions, such as the impromptu romance of the waltzer boy and monkey daughter (Aimée-Ffion Edwards) or Mrs Twit's Yuletide outburst over children: "A shy child…is nothing more than a pretentious bed-wetter; and is there anything more atrocious in the world than a confident, expressive, flamboyant child?"

The world is turned upside down, sort of, and there are hello-Dahly references to the wormy spaghetti, Mrs Twit's giant stick, face-bashing with frying pans, the hugtight glue (though the effects of this are not nearly sticky enough) and the slithery frog which Twit shoves down his wife's jumper, not up her nightdress as in the book.

Walsh has "mischievously adapted" Dahl but he can't translate the grunginess and sheer liberating filth of the prose into action; and he's written mimetic stage directions that the actors and movement director, Steven Hoggett, fail to animate on a static and overcrowded playing area.

The show is encased by its own limitations, and the final stand-off, with Yorkshire Terrier Man telling the Twits that he and his crew have what they can never have – togetherness, friendship, etc – sounds like Harry Potter squeaking at Voldemort before the final duel. Off they jolly well go, to lose themselves "in the joy and love and community" of their fellow countrymen and women. After you with the sick bag, Mrs Twit.

The Twits runs at the Royal Court until 31 March.