Opportunities to see Peter Shaffer’s well-crafted 1979 play Amadeus are comparatively rare and should be seized when offered. While some might baulk at what is, in essence, a dramatised libel on the Viennese court composer Antonio Salieri – and to an extent on his downtrodden rival Mozart – the piece is cleverly constructed and almost always gripping, with the added benefit of a brilliant (all-Mozart) score.

This week’s production in the sympathetically intimate setting of the North Wall in Summertown, from the semi-professional group Green Bear, is an intelligent, workmanlike affair which reveals the many strengths of the drama while not disguising its weaknesses. Chief among the latter is its prolixity. Milos Forman managed to trim the action to a shade under two hours for his Oscar-winning film adaptation of 1984; here it runs to a full three, with interval.

For much of the time we are being addressed by Salieri who, as with Iago in Othello, is the ‘villain’ supplied with the largest role. In a series of long speeches to the audience he lays bare his poisonous envy of his rival on the Viennese music scene (though ‘rival’ is what the all-powerful Salieri never lets Mozart become) and his fury at a God who could equip so graceless a “creature” (his word) with so divine a gift.

Prasanna Puwanarajah plays this difficult part extremely well. He must remember, though, that in adopting a hushed conspiratorial tone for these confidences he would do well to let them be heard beyond the confines of the first six or seven rows. Madeleine Dodds, otherwise first-class as Mozart’s wife, Constanze, also needs to speak up, especially in her long speech as the play nears its end.

There is certainly no problem on this score from Richard Graylin (pictured) who gives a bold, bravura portrait of Mozart as a giggling, raspberry-blowing vulgarian – which, sad to say, appears to be one indubitably accurate representation in a play long criticised for its loose approach to the truth. There is exceptionally fine work, too, from Andrew Goddard as Baron Gotfried van Swieten, a serious-minded aristocrat offended by the young composer’s flippancy and, as he sees it, betrayal of Masonic secrets in The Magic Flute.

Himanshu Ojha and Tom Devine do valuable double duty as acidulous commentators on the action and a pair of aristocrats in the courtly circle about Emperor Joseph II. This merry, and rather silly, monarch is played very well by a young actor called – and this is no joke – Amadeus Stevenson.

Amadeus continues at the North Wall until Saturday. Tickets, telephone 01865 319450.