The chestnuts are roasting just outside the gates of Hyde Park and inside the Royal Albert Hall the family McGarrigle, or Wainwright, depending on which way you want to look at it, that dysfunctional lot, are gathered for a festive sing-along. Kate McGarrigle – mother, writer of haunting, plaintive folk songs – is the linchpin tonight, and that's probably how it should be. It was McGarrigle, after all, who nurtured Rufus and Martha Wainwright after the breakdown of her marriage to the folk luminary Loudon Wainwright III.
As with all festive gatherings worth their salt, the latch is left off the door for the half-cut aunts and uncles and whoever else to wander in. French and Saunders – dutifully silly – prepare the stage for the McGarrigle throng, led by Rufus, gleefully waving a Prom-issue Union Jack. Together, they prance and promenade and carol their way through the traditional "Seven Joys of Mary". Boy, it's camp in here.
The guests, with gusto, earn their places at the table. Elbow's Guy Garvey, at the tail end of a victorious year, croons his way huskily through Joni Mitchell's "River"; the wonderful Ed Harcourt joins Martha and has a suitably ragged pop at Shane MacGowan in "Fairytale of New York"; and Boy George soulfully conjures the old magic in a ska "White Christmas".
Not everything quite comes off; but crucially, the whole evening holds firm at the centre. McGarrigle, with her sister and musical partner, Anna, oozes rootsy brilliance and Martha, endlessly expressive, dazzles under the spotlight with a slight whiff of Piaf on a glittering "I'll Be Home for Christmas". But it's Rufus who really takes the breath away, first on his own "Christmas Is for Kids", sounding a note of regret on the passing of the years, and then in the evening's enchanting zenith, as the microphones are turned off and it's left to his stark, resonant voice to woo the audience in a French serenade.
There's enough time for Garvey to bring the Albert Hall to its feet for a sway-along "Happy Xmas (War Is Over)", for Kate and Anna to squabble over whether angels have sex and for Martha to announce that three weeks previously, Mary-like, away from home, taken in by a stranger (the NHS), she gave birth to a son. A Christmassy ruse, perhaps, in other hands: but then you get the sense that in this family that kind of thing probably happens all the time.