The arrival of Robert Louis Stevenson in the West End marks an early beginning to the season of festive family entertainment. Pundits predict that, credit crunch or not, it’s going to be a bumper year for theatrical escapism. Nonetheless, parents down to their last few pieces of eight should think twice before spending them on such an underwhelming account of piratical swashbuckling, which even Keith Allen’s anarchic spirit fails to perk up.

Allen’s Long John Silver at least injects some life and colour into the largely inert proceedings, arriving on the scene at Bristol Docks with a huge wooden prosthesis, mechanical parrot, jaunty headgear and gold hooped earring.

You feel that Allen is itching to let rip on all this prototype Boys’ Own adventure nonsense — there are just two women on offer here, in the tiniest of supporting roles — but he is constrained by the uncertain tone of Sean Holmes’s direction. Like an ill-rigged mainsail, the production strains in a number of directions simultaneously, tacking now towards straight drama, now comedy. Even when mutiny strikes the good ship Hispaniola, there is a dreary sense that nothing is actually at stake.

It’s easy to forget how dense and, often, convoluted, Stevenson’s original text is and Ken Ludwig’s adaptation certainly doesn’t stint on narrative confusion. It’s clear enough that young Jim Hawkins (Michael Legge) is plucked from a quiet life at the Admiral Benbow Inn because of a map purporting to indicate buried treasure over the seas, but the ever-shifting on-ship allegiances are much harder to follow.

Lizzie Clachan’s design involves an impressive amount of rigging strewn around the stage and a platform for three musicians to be wheeled about on, but dismally tenebrous lighting doesn’t do much for the half-hearted video projections.

Legge starts off decently, giving Jim a wide-eyed innocence, yet crucially fails to suggest a boy hardening into manhood through a series of challenges and betrayals from Silver’s ambiguous surrogate father figure. In the supporting cast, Tony Bell makes a pleasingly bluff Captain Smollett and John Lightbody stops Squire Trelawney just short of all-out pantomime camp. Even so, this Treasure is chocolate coins rather than real gold.

Booking to 28 February. Box office: www.treasureislandtheplay.com and 0845 481 1870.