Tristan und Isolde became Tristan und Isolden (if that’s the correct plural!) at Grange Park on Saturday, there being two singers playing the heroine of Wagner’s great opera. Alwyn Mellor, who had earned enthusiastic reviews for her performance earlier in the run, had a problem with her voice, and confined herself to miming the actions while vocals were supplied from the side of the stage by Susan Bullock.

Since Ms Bullock is widely regarded as Britain’s leading Wagnerian soprano it will be seen that the audience was getting a good deal. As the opera festival’s chief executive Wasfi Kani said: “We are amazingly lucky. Two for the price of one.”

As Wasfi said would be the case, one soon grew used to the situation, scarcely noticing whence the sound — often a truly wonderful, sound of heart-stopping beauty — was emanating. It did cross the mind from time to time, though, that Susan Bullock, who is married to this production’s Tristan, the excellent Richard Berkeley-Steele, must have thought it a little strange to be watching so closely as another woman grappled amorously with her spouse.

This is a sensationally good production on a scale that could hardly have been imagined when GPO was set up by Wasfi as recently as 1997. The glories of the score were thrillingly explored on Saturday under conductor Stephen Barlow, with the English Chamber Orchestra on superb form from the opening bars of the prelude to the climactic close of the Liebestod nearly six hours later.

The performance was marked by top-class singing across the cast. Sara Fulgoni gave an affecting portrait of Brangäne, the supplier of the fatal love potion, while Tristan’s retainer, Kurwenal, was presented with great power by Stephen Gadd.

Firm favourite with the audience, though, was bass Clive Bayley, as the cuckolded King Mark. His long philosophical soliloquy in Act II was a tour de force, fully justifying the acclaim he received at the final curtain.

Director David Fielding doubled as designer in this modern-dress production, with Act I on the deck of an ocean liner packed with the coffins of the Cornish dead. One might have managed well without the too literal giant goblet, knife and skull that intrude into the bedroom of Act II, but the beauty of the Britanny seascape of Act III — admirably lit by Wolfgang Goebbel — could hardly be denied.

Until July 3. For information on tickets, call 01962 737360.