Crosby, Stills and Nash, Hammersmith Apollo, London

As the trademark three-part harmonies soar into "Carry On/Questions", "Marrakesh Express" and "Long Time Gone", everyone present is reminded why - on hearing Nash's voice on top of his and Stills' for the first time in 1968 - Crosby described the sound as "the rightest thing I've ever heard".

But soon the "greatest hits tour" promised on our ticket stubs has given way to a mixed bag of solo material that no one has really come to hear. As the first half progresses, it also becomes increasingly clear that Stills - who has largely been the missing piece of the puzzle in the 14 years since the trio last visited the UK - is not having the time of his life. He cuts a twitchy, fidgety and uncomfortable figure and only really springs into action to wring the life out of "Old Man Trouble", the grizzled-bluesman moment from his current stinker of a solo album. From the same record, we are also treated to "Feed the People", with its lyrics straight out of The Collected Political Speeches of Sir Bob of Geldof. Even in Live8 week, the chorus of "Why not feed the people everywhere/ And let the peace begin" fails to produce the required singalong.

Having spent over 30 years forgiving Nash his own too-twee tendencies, this is almost too much to bear for those of us who have come expecting Stills to provide a much-needed edge. So it's left to Crosby - the reformed jailbird and junkie who required a liver transplant to save his life in 1995 - to effortlessly and egolessly steal the show.

The second half opens with his voice of a thousand angels joyously parping "Helplessly Hoping" and "Guinevere". By this stage, Stills has become a ghostly bystander and Nash is left to Brit banter with the audience and provide those pitch-perfect harmonies. For while Crosby's life story may read like a psychedelic soap opera, the walrus of peace and love has emerged on the other side, a beaming picture of health with as many children as chins. Tonight's musical highs are all down to him - with "Almost Cut My Hair" in particular as paranoid, angry and haunting as the day its freak flag first flew.

Of course, the CSN banner has always been a catch-all brand-name for three very different singer-songwriters. "Nash writes the anthems everybody loves. Stills writes great rock'n'roll. And I do the weird shit. It's a tough job but I'm unusually suited to the task," Crosby tells us tonight. And thank the lord he is. Because by the end of the show, Stills' lead vocals on "Wooden Ships" are being sung by Nash and there is no attempt to play either of the crowd-pleasers that require his lead vocals: "Suite: Judy Blue Eyes" and "Woodstock". But while Crosby is in this form, the CSN juggernaut can roll on a little longer yet. Though, until they all save themselves, they may have to leave that tricky saving the world stuff to the next generation.