Like heavy metal, R&B has recently been transformed. Once the province of toe-curling balladry and bump-and-grind innuendo, it has reinvented itself as thrillingly futuristic pop music, an apparently bottomless source of creative, brilliant hits. Yet, while metal has renounced its naff, spandex-clad past, R&B seems unable to resist the lure of schmaltz.

Destiny's Child, the world's biggest-selling R&B artists, are a perfect example. One minute they release a fantastic single, the next make an abominable Christmas album. Tonight this lack of consistency is in plentiful evidence. Surrounded by pyrotechnics, the trio launch into their biggest hit, Independent Women. For half an hour the pace never lets up. One hit breathlessly follows another: No No No, Bug-a-Boo, Bills Bills Bills, Get on the Bus. It is a superb opening. Anyone searching for evidence of Beyonce Knowles's allegedly dictatorial rule over Destiny's Child may note that whenever Kelly Rowland or Michelle Williams takes a vocal, Knowles upstages her with a diverting shower of pelvic thrusts.

But the show sags in the middle. Clad in sparkling gowns, each member of the band sings a ballad. Like all R'n'B ballads, they slither torturously along, with the pace and unctuousness of a slug. Each comes with show-offy trills and embellishments that make a syllable seem to last a week.

Once again, you are left in no doubt that Knowles in charge. The others are demonstrative when they sing but, in Dangerously in Love, Knowles virtually enacts Ophelia's mad scene, rushing from one side of the stage to the other, her eyes bulging and frenzied. As a finale, she drags out the word "dangerously" until it dies of old age. The trio talk to the audience: "London, England!"

"I said, London, England!"

"London, England, we are so excited to be here. We are ready to be here in London, England."

"We are definitely excited."

So is London, England. They cheer everything, even a disheartening drum solo.

Still, there is plenty to cheer about when Destiny's Child return to the dizzying format of the show's opening: Survivor, Jumpin' Jumpin' and Bootylicious. Another burst of pyrotechnics and they are gone. It is perfectly staged, brilliantly sung, and slightly overwhelming. Overwhelming enough, in fact, to counteract the trio's penchant for sickly excess.