Authenticity is an awkward sort of beast to bridle. Period costumes? Period styles of playing? Period instruments? It’s a tricky theatrical problem for which the Bury St Edmunds Theatre Royal have come up with a neat solution which actually works. The piece in question is the 1783 ballad opera The Poor Soldier with words by John O’Keeffe (he of Wild Oats fame) and music more-or-less by William Shield.

The cast of eight, with one actor doubling roles, plays the score on stage in between speaking and singing. Often a character is instrument in hand during dialogue or duet, providing his or her own accompaniment to the succession of easy listening numbers. It gives a verve to the whole escapade, not necessarily lyrically smooth but always tuneful and lively – a country equivalent of The Beggar’s Opera as it might have originally sounded before Austin or Bliss or Britten gentrified it or singspiel before Mozart, complete with final vaudeville.

We are in Ireland just after the end of the War of American Independence. Darby and Dermot, two country lads, have an eye to Kathleen, the ward of the parish priest. His niece Nora also has suitors – a gentleman, Fitzroy (can his intentions be honourable?), his French valet, Bagatelle (ditto) and Patrick, a young soldier just back from the war who is rich in honours but poor in purse. Father Luke’s instrument is appropriately the bassoon, Kathleen favours the clarinet and Nora is accompanied by her own flute.

Musical director John Rigby stirs this stew with the lightest of off-stage touches; his performers look right for their parts, act extremely well and play with considerable accomplishment. When the curtain rises after the prologue it reveals the players as orchestra for the potpourri overture within a box setting by Mia Flodquist. In the distance is the Great House, behind the players an apple tree bears fruit and its fallen bounty masks the footlights. Director Colin Blumenau balances the conventions of the genre with a modern audience’s expectations very neatly.

Sioned Saunders is a charming Nora singing her second-act lament particularly well, though Elizabeth Reid’s Kathleen is by no means put into the seconda donna shade. Daniel Summers is our carrot-topped hero with Dominic Gerrard making Fizroy’s change of heart and conscience at the end thoroughly credible and gives a good rendition of his first act hunting song. Neil Salvage as Father Luke has only one song but both Tarek Merchant as Dermot and Bagatelle and Sam O’Mahony Adams as Darby have a lot to make a song-and-dance about. Great fun for all concerned on both sides of the footlights.