Porgy and Bess is the most glorious score and Trevor Nunn’s Glyndebourne production 20 years ago was simply magnificent. Yet the new “musical” version of the collaboration between George and Ira Gershwin and librettists DuBose and Dorothy Heyward at the Savoy is a disappointment. Decently sung but unconvincingly edited, the show falls between several stools, not quite opera, not exactly operatic musical. It’s a lowbrow hybrid, or highbrow lowbred.

Not until the first half-hour of the second act – the piece has been cut from three acts to two – is there any sense of musical continuity or texture, as at last we travel from the picnic on Kittiwah Island – where the coke dealer Sportin’ Life (an underpowered O-T Fagbenle) sings “It Ain’t Necessarily So” - back to Catfish Row in South Carolina, where Porgy exacts his murderous revenge on Crown for loving his woman, Bess.

The show doesn’t carry the overwhelming impact of Nunn’s Glyndebourne version, which was received with rapture and standing ovations. The impulse to make a coherent musical theatre evening is obviously a good one, but the show now seems too skimpy a version of the opera. And the effort of jazzing it up with dance sequences, right from the start, trumping “Summertime” with a hectic ensemble number, is a stunning move in the wrong direction. Nunn claims that Gershwin always intended to write a jazz prologue. Musical supervisor Gareth Valentine has done it for him, but it feels slightly forced.

Nunn is a master of bringing a community alive on the stage, but the detail seems plastered on to both the major performances and John Gunter’s design. Organic is not a word I like all that much as a critical term, but organic is what the show does not feel like.

The other big problem is the sound system. I never felt I was hearing the voices raw and true. (When is the musical theatre audience going to finally stand up and be counted on this subject?) There’s a microphoning conduit that gets in the way all the time, and the Gershwin blues are deflected through an electrical membrane that is off-putting, though not as bad as in Nunn’s otherwise joyous Anything Goes.

Clarke Peters is an anguished, tortured Porgy who relaxes at last into “I Got Plenty o’ Nuttin’” and sings a beautiful light baritone counterpoint to Cornell S John’s tremendously threatening bass Crown. On the other apex of the romantic triangle, Nicola Hughes is a technically powerful but curiously unengaging Bess. The continuity of the score is so broken up in Valentine’s musical adaptation that you lose that crucial sense of an overall musical journey that the full operatic production can supply. The storm scene seems arbitrary rather than inevitable; Clara and her baby come and go without making any real narrative impact.

This should have been the great restorative event of the musical theatre winter, but it isn’t. Certain performers, apart from Mr Peters, stand out: Dawn Hope as a serene Serena, Lorraine Velez as Clara, Ruby King as the strawberry woman. But Catfish Row never buzzes with the animation of Nunn’s Tennessee Williams or Chekhov productions, and the defiant exit of poor old crippled Porgy in pursuit of his lost Bess is an act of manufactured hysteria rather than deep, solemn dedication.

- Michael Coveney