It’s a two star but for a while it felt like a four. Tim Riley and John Lovat’s very silly reworking of the Barber of Seville sets it in an Ealingesque 1950s. It gets off to a cracking start. On a conflated, cartoon set Count Almaviva, renamed the Marquis of Basingstoke, creeps on to a jazzy, walking bass, and woos Rosy Simmons, ward of the tailor Mr Bart, to a washboard accompaniment.

For Opera Unexpected are all about irreverence and accessibility. The music, a jazz trio plus saxophone under the baton of Stephen Hose, gives you the basic orchestral lines played with verve. Dialogue replaces recitative.

The characters are reassuringly familiar. Richard Immargluck’s Figaro is a strangely well-spoken teddy/wide boy, Patrick Ashcroft’s Marquis a Woosterish fop. The policeman says: “Evening all.” Angry old Mr Bart says: “I don’t believe it.” The hoary jokes almost double bluff your expectation, surprising you into laughter.

It’s deliberately shambolic, and Immergluch and Ashcroft are excellent comedians, charming their way through this pantomime shtick, helped by a hilarious Dame turn from Philip Canner as bass nun Donna Basil and the improvisations of Tony Harris as the local plod. It’s all very fun. Then bearable. Then not so.

It is the music that breaks the spell. This adaptation has cut down near four hours of Rossini to two and a half, and should have cut an hour more.

For this opera is a hard sing, its arias long and packed with pyrotechnics. The director tries every trick to distract attention from voices that are at times nowhere near keeping up. But the barrel is empty by Mr Bart’s aria — seven minutes of vocal show the gallant performer can’t make interesting.

After that, things begin to spoil. Bad jokes seem just bad jokes. Zany irreverence seems self-indulgence. With wine at the interval it might have remained hilarious. Sober, all I took away was a worn sense of irony and a paradoxical lesson: with “so bad it’s good”, less is quite definitely more.

Until 15 April. Information: www.warehousetheatre.co.uk.