I have always been a bit of a barbarian when it comes to Swallows and Amazons. This children’s classic from 1930 isn’t hard to navigate like Moby Dick, I know. Yet on all my failed attempts to set sail with John, Susan, Titty and Roger Walker for their summer-holiday escapade, I’ve let myself get bogged down by Arthur Ransome’s prose.

No longer. This musical play by the writer Helen Edmundson and the songwriter Neil Hannon (aka, the Divine Comedy) went down so well when it opened at the Bristol Old Vic last Christmas that it’s now docked in the West End briefly before embarking on a tour next year. And what a wonderful piece of theatre it is. As the adults playing these 7 to 12-year-olds stride around in their knee-length shorts preparing for their island jaunt, you melt — because they’re singing a song about facing the unknown, expressed in lines about packing the corned beef. And by the time they’ve set sail in Swallow, with castmates extending blue ribbons on either side of the boat to suggest — sensationally well — its progress through the water, you can’t help but climb on board.

The director Tom Morris uses overt stagecraft to co-opt our imagination here, as he did when he co-directed War Horse. The supporting actors also play instruments — strings and piano, mostly — and work the props. As the two Northern pirates from the rival boat, Amazon, ululate their way through the stalls, as Polly the parrot gets represented by a feather duster and a pair of secateurs, we always know that what we’re seeing is all let’s pretend.

Which is the perfect way to involve us in a story that’s all about let’s pretend. There is plenty of wry humour here, but Morris never allows it to undermine our sense of wonder. Indeed, there is a delicious sense of danger ahoy at the end of Act I as James Farncombe’s lighting dims on Robert Innes Hopkins’s set and the two sides declare war.

If the second half isn’t as thrilling as the first suggests it will be, and indeed if two-and-a-half hours is a few minutes too many, the home-made sense of daring endures and the cast of 13 do full justice to Hannon’s smart songs. Richard Holt nails the way that 12-year-old John goes from captain-of-school calm with children to unease with an adult; burly Stewart Wright milks the part of innocent little Roger for all its worth but not a drop more; Katie Moore is a treat as sensible Susan; Akiya Henry is plucky but never abrasive as Titty; Celia Adams and Sophie Waller are adorably unfriendly pirates.

So it’s a thoroughly entertaining, beautifully balanced show for all ages: sincere but not strait-laced, playful but not facetious. Maybe 85 million readers aren’t wrong after all.