“It’s not just pork, Gilbert. It’s power.” That’s the enabling quality of the sow with long lashes in Betty Blue Eyes, the most thoroughly English and charming new musical in the West End for a very long time, based on Alan Bennett’s hilarious 1984 film, A Private Function.

The star herself is a pretty porcine too good to eat, controlled animatronically by stage management and mostly confined to a tin bath in the second act of the show, once Gilbert Chilvers (an outstandingly sweet and light-on-his-pins Reece Shearsmith), the village chiropodist, has snaffled her down on the farm.

Betty’s being reared for a banquet to celebrate the royal wedding of 1947. In case we didn’t know it was coming, Richard Eyre’s production starts with newsreels and public announcements (“Eat More Greens, Less Bread”) at a time of post-War rationing. The pig’s a forbidden luxury, but the town council wants to impress 150 visitors.

They have to contend with the spoilsport interference of Adrian Scarborough’s frenetic little meat inspector, not to mention the social ambitions of Gilbert’s wife, Joyce, played by Maggie Smith in the movie and here given an added musical theatre leading lady status by the marvellous Sarah Lancashire.

The adaptation is the unlikely work of two Americans, Ron Cowen and Daniel Lipman, who found their way to the writing team of George Stiles (music) and Anthony Drewe (lyrics) thanks to Godspell composer Stephen Schwartz.

Producer Cameron Mackintosh has put the whole thing together with splendid panache, Eyre’s fine work supplemented by the fluently conceived, picturesque designs of Tim Hatley and the witty choreography – making the most of queues, victory marches and processions – of Stephen Mear.

Bennett’s story line is faithfully followed and many of the favourite lines of Joyce (“I’m going to throw caution to the wind and have a sweet sherry”) retained. But the ending is changed for musical comedy purposes, very much in the spirit of the adaptation: the best numbers take the characters out of their present and into their fantasies, and in that respect the musical is most like Billy and, for that matter, Billy Elliot, two of its stable-mates in the best of bulldog British musicals category.