Alison Krauss and Union Station Royal Festival Hall, London

Standing stock still behind their simple microphones in the middle of the huge Royal Festival Hall stage, Alison Krauss and Union Station looked the epitome of the country cousins come to town. Don't believe a word of it.

Though they played up to the image with a few cracks about only expecting to fill a few rows (of a close-to-sold-out house), these are folk who should be well used to success. Now That I've Found You, the anthology album that this tour was in support of, has spent the past few months setting milestones for sales of bluegrass records as well as crossing over into the pop charts and gaining airplay on mainstream radio. And Krauss herself - though still only 24 - already has a string of releases to her name and has been inducted into the Grand Ole Opry.

Not that any of this should give you the impression that this is yet another of that current crop of country artists who are largely rock performers with big hats and lots of rhinestones. Though generally categorised as bluegrass, Krauss and her long-serving band actually defy easy pigeon- holing.

As on the record, Saturday night's nearly two-hour show moved with total ease between a variety of styles: from the album's title track, originally a soul number whose British co-writer John MacLeod took a bow during the performance, through the band's mainstay of what they called "pitiful love songs" to Bad Company's anthemic "Oh Atlanta".

Such a spread of influences could easily result in a mess. But the appeal of Krauss and her cohorts lies in their ability to use their technical skill on acoustic instruments to put their own complexion on whatever they perform. With a pure voice and fiddle-playing that shifts effortlessly from the joyous to the mournful, Krauss can be spellbinding on her own.

Thankfully, however - given her somewhat gauche comments between numbers - she is not a performer with a backing band. This is ensemble playing at its best, where each of her four accompanists - whether on guitar, banjo, mandolin or upright bass - relishes the interplay with the others.

As such, they are likely to have a place in the hearts of those who appreciate the simple virtues long after some of country's raunchier bigger names have ridden off into the sunset.

ROGER TRAPP