The Rise and Fall of Little Voice/My Real War 1914 - ?, The Vaudeville Theatre/ Trafalgar Studios, London

There may come a day when you will have to have featured in a viewer-voted TV talent contest to do anything of note in London theatre – not just star in a West End show, but direct Shakespeare, run a flagship national company, the whole shebang. So I suppose we should be relatively calm about the containable fact that an X Factor finalist, Diana Vickers, is currently failing to add either lustre or plausibility to Terry Johnson's energetically mediocre revival of The Rise and Fall of Little Voice, Jim Cartwright's lovely blowsy-yet-poetic Northern tragicomedy which was first seen in a production by the young Sam Mendes at the National in 1992. The blame lies not with Ms Vickers, but with the misguided producers.

She portrays the eponymous Little Voice, a lonely, bereft young girl who has retreated to her bedroom with her beloved dead dad's stack of diva records, and into her world of eerily spot-on impersonations of these icons. Downstairs, her frustrated, motormouth slattern of a mother is throwing herself at anything in trousers. When Jane Horrocks took the part of "LV" for Mendes (and in the later movie), she was piercing. Her Little Voice seemed to carry round her own micro-climate of withdrawn grief and achingly suspended potential.

The voices were her patrimony and her amulet against a hostile world and they seemed to possess her with an uncanny mediumistic power. Vickers, by contrast, is just an inexperienced performer who has been taught to do a passable Shirley Bassey and Cilla Black, but whose Judy Garland has none of the right needy, neurotic throb. She looks mighty fine in her silver, slit-up-the-side number in the spectacularly lit climactic concert, but where Horrocks was riding a vortex of rapturous ventriloquism, Vickers is merely meandering through a medley. It doesn't help that, as her comic-grotesque mother, that lovely, sensitive actress Lesley Sharp has been grotesquely miscast.

The speciousness of this exercise is summed up by the new song specially written for this production by Take That's Mark Owen. It's supposed to show Little Voice capable of singing in her own right for the first time. But it makes no psychological sense. How could a girl brought up entirely on daddy's records of show- and cabaret-tunes possibly start to sing in this standard-issue sob-holler contemporary-pop mode, such as can be heard on umpteen X Factor auditions. It would take more than a movement of her spirit; it would take a complete brain transplant for this dismal dirge to sound right as the spontaneous self-expression of the newly liberated heroine.

If he had lived, the hero of the one-man show My Real War 1914-? would have become the uncle of John Le Mesurier, the actor best remembered for his role as Sergeant Wilson in Dad's Army. But 2nd Lieutenant Havilland Le Mesurier – known as "Lem" – was killed in action in 1916, leading an assault by the men of the King's Royal Rifles on a German trench. He was 22.

Lem's privately published letters to his parents have now been woven into a poignant monodrama by Tricia Thorns of Two's Company, an outfit whose excellent "Forgotten Voices from the Great War" project was started in protest against the US/UK invasion of Iraq in 2003. Pukka and puppyish with his lanky limbs and large china-blue eyes, Philip Desmeules delivers a touching portrait of this stiff-upper-lipped scion of the Channel Island gentry.

The Rise and Fall of Little Voice: (0844 579 1940) to 30 January; My Real War 1914-?: (0870 060 6632) to 31 October