Sugababes, Hammersmith Apollo, London<br></br>The Knife, The Scala, London

The appeal of Sugababes, in their early days, was precisely that they appeared to be an authentic London girl gang, the antithesis of those manufactured, stage-school pop groups. Six years is a long time in pop, and a revolving-door personnel policy has meant that the life expectancy of a Sugababe is roughly equivalent to that of the average Southampton FC manager.

First to fall was Siobhan Donaghy, who was frozen out in August 2001, to be replaced by Heidi Range, a former Atomic Kitten. Then, late last year, the cock-eyed, sullen, gorgeous Mutya Buena, having sworn she would never leave, did a U-turn and quit. Her replacement, picked by management, is 21-year-old Amelle Berrabah from Aldershot.

A friend of mine, not known for his political correctness, refers to the Sugababes as "darkie, whitey and inbetweeny". And you can't help wondering if the band's management had similar criteria when picking the Philippines-descended Buena's successor.

The upshot is a ridiculous situation whereby Keisha Buchanan now fronts an act calling itself "Sugababes", in much the same way that Axl Rose now fronts something called "Guns N'Roses". But am I the only one who cares? The Apollo is sold out two nights running and my doubts are drowned by screams. And in some ways, the survival of this band should be applauded. From the start, they were always one of the more interesting pop acts, launching themselves not with some piece of shiny sub-Stock Aitken Waterman inanity but with the sultry, thoughtful "Overload".

Sure, they're guilty of some dire balladry, and there's much quiver-lipped warbling tonight, particularly during the medley of "Shape" and "Stronger". Sure, they can be trite. But they're also responsible for "Freak Like Me"; the sublimely Abba-esque "Push the Button"; and "Overload" itself. Furthermore, The Supremes recorded some of their best stuff after Diana left. Who's to say Sugababes can't do likewise?

As their set reaches its inevitable conclusion with "Red Dress" (the costumes are predictably literal), I wonder what's going through Keisha Buchanan's mind. When she looks left and right and sees two strangers, does she smile inwardly. Or does she tremble that "It'll be me next"?

"We are the people who's come here to play/I don't like it easy/I don't like it the straight way". In her broken Swenglish, Karin Dreijer isn't messing. For one thing, The Knife - in their first-ever full gig - don't even perform the song from which those words are taken, "You Take My Breath Away". But even if they did, the Stockholm synth duo (the other member is Karin's brother Olof) could never be accused of taking the path of least resistance. This, in short, is one of the strangest shows London will see in 2006.

The Knife first caught my ear with the 2004 album Deep Cuts, a suite of gorgeously melancholy electronic pop. Their latest, Silent Shout, is altogether colder, darker and more difficult. The live show is unmistakeably in the vein of the latter.

The song for which they're best known, "Heartbeats" (covered by Jose Gonzales and used in Sony Bravia ad) is reworked in a bass-booming style, and "Pass This On" is reinvented with a steel drum. By the time they end with "Silent Shout" itself, the Scala's collective eyebrows are through the roof.

If The Knife had made their live debut 18 months ago, I can't help suspecting we'd have witnessed one of the most wonderful pure pop concerts imaginable. What we get is, in every sense, something else.

s.price@independent.co.uk