Martina Topley-Bird, Regent's Park Open Air Theatre, London

An amphitheatre. In Regent's Park. In the rain. Could the setting be more perfect for a young man with a Jeff Buckley falsetto so sweet it could rot your teeth? Tom Baxter, in an open-air theatre more suited to productions of A Midsummer Night's Dream, exhibits a vocal impersonation of the balladeer so uncannily nuanced it must have Starsailor's James Walsh weeping over his copy of "Mojo Pin". Warming up for the night's main act, Baxter unfortunately overarches himself with a gluey song with the ill-fated title "Almost There". Hopefully, like Oberon, he'll wake up with the right woman one morning and get over it.

An amphitheatre. In Regent's Park. In the rain. Could the setting be more perfect for a young man with a Jeff Buckley falsetto so sweet it could rot your teeth? Tom Baxter, in an open-air theatre more suited to productions of A Midsummer Night's Dream, exhibits a vocal impersonation of the balladeer so uncannily nuanced it must have Starsailor's James Walsh weeping over his copy of "Mojo Pin". Warming up for the night's main act, Baxter unfortunately overarches himself with a gluey song with the ill-fated title "Almost There". Hopefully, like Oberon, he'll wake up with the right woman one morning and get over it.

Comparisons with the Bard's comedy come in useful when pondering Martina Topley-Bird's somnolent, bewitching voice, which sounds almost supernatural on "Intro", tonight's opener, and the first track on her Mercury music prize-nominated debut album, Quixotic. That album - four years in the making and sporting impeccable producing credentials (David Holmes et al) - was expected to eject Topley-Bird to Björk levels of superstardom. It has, however, so far failed to capture the imaginations of those who enjoyed her Billie Holiday to Tricky's Miles Davis on Maxinquaye, the Bristolian trip-hopper's magnum opus.

After an imperious entrance in a scarlet kimono and floral headdress to the strains of a bluesy harmonica, the 29-year-old tops her support slot's vocal pyrotechnics in an a capella spiritual sung through a crackly vintage microphone; it sounds like something recently risen from the grave. Part African-American, part Seminole Indian, part El Salvadorean, Topley-Bird exudes an exotic, aloof charisma, and boy, she looks good with a guitar. For "Too Tough To Die", Quixotic's swamp-rocker, Topley-Bird dons a scarlet axe - to match the dress, natch - and does a great PJ Harvey fem-rawk swagger along to the infectious riff. But the next track, the gospelly "Soul Food", is an icky ode to romantic release. "Ragga", produced by Tricky (who also duets on the album), injects welcome anger and sexual deviance with its references to baseball batting anyone with the gall to "come round/dumping fun around", and relishing "doing things I wouldn't normally do", but new track "Poison", with ska and calypso inflections, is unmemorable, as is "Lying" - no more stimulating here than on the album. Then a ho-hum Toots and the Maytals cover, an engagingly mournful "Lullaby", a muted "Need One" (performed spunkily on Quixotic with Queens of the Stone Age), and an encore which included the overblown - and badly punctuated - "Stevie's (Day's of a Gun)". A cute coda featuring the Fingersnaps, three winsome little girls who provided backing for a cover of the Shangri-Las' "Remember (Walkin' In The Sand)", was one for the papoose-laden new mums in the audience.

As on Quixotic, Topley-Bird tries too hard tonight to do too many things - soul, blues, rock, gospel, world music, bubblegum pop - and this magpie, often maverick approach feels diluted by a tendency toward Morcheeba-lite digestibility. It's a frustrating compromise, in which the only thing that isn't in question is that voice.