Saturday night was Cup Final day in London and - in passing - the last time the crowd joined for "Abide With Me" at Wembley. A fact that sprang to mind later in the evening when - at the Festival Hall - Joni Mitchell overcame a concert that threatened to culminate in disaster - by moving the sell-out audience to their feet for a community version of "The Circle Game".

The song was in fact Joni's second encore at the end of a performance that proved she has a magic about her that can overcome a series of accidents destined to throw most artists into marching straight off stage again. All artists are aware when they don't give their best or when situations are not conductive to a build-up of atmosphere and perfection, and at the end of Saturday night both Joni and Jackson Browne were sad and disappointed people - although both were obviously moved by the audience reaction which warmly, overlooked all the aggravation and holdups.

The evening had started badly - half an hour later than advertised the audience were let into the main hall having been barricaded out. Apparently the sound system had been playing up all day, and the sight of road managers and sound guys still fiddling around with leads as we got to our seats was hardly a settling vision. Jackson Browne was making his second London appearance (last time in he appeared with Laura Nyro). Out of his 40 minute set only three songs actually got completed - "Jamaica Say You Will" (after two attempts), "These Days" and "Jesus In 3/4 Time". For the rest he was continually cut off the mike and even when he moved to piano found the sound didn't work at all. It says much for Browne that he managed to not only draw the audience's sympathy but kept his temper and proved that with a sense of humour almost anything can be overcome. Almost...called back for an encore he was forced to simply stand on stage and smile, the sound having finally packed up for good.

During the interval the Festival Hall's own system was moved onstage for Joni who came on looking calm and beautiful - even though earlier she had been standing in the wings puffing nervously at a cigarette as Jackson was onstage.

In beige silk trousers and shirt, her long blonde hair hanging round her shoulders, Joni had the appearance of a creamy snow princess. But her fragile look and personally identified songs belied the true tougher performer underneath. She was obviously aware of a lack of clarity through the speakers, but she went through her set with an assurance and delicate charm, chatting warmly to the audience, telling stories about her voyage to Greece with her friend Penelope and sending herself up. And an audience that could often hardly hear what was emerging lyrically but could only hear the resonance of that voice that can suddenly twist itself round inside itself, sat hushed and on needle tip trying to catch every little piece of magic. For this appearance in London, Joni aired a lot of regular favourites "Big Yellow Taxi", "Woodstock", "You're A Mean Old Daddy", "Both Sides Now", and "Blue" moving from guitar to piano to dulcimer. But many of the evening's highspots were provided by brand new material like "Oh Honey You Turn Me On - I'm A Radio", the slightly bitter attack at her own fame "Through The Roses" and - certainly for me the most beautiful song she's ever written - "Song For Ludwig" which caught the audience mid-breath.

Not many artists could have come off against the odds as Joni did on Saturday. It says much for her as a professional artist. Her arrival back onstage with Jackson at the end for "Circle Game" proved that not only is she a lady but that she has a real sense of fair play.