French pop music in Britain is like British food in France: either ignored or sniggered at. The exception that proves the rule is Jane Birkin, who filled the Festival Hall for an evening of French song last Tuesday. And she is English.

Well, sort of. Her beauty has the girlish simplicity that is seldom found outside the smarter parts of Paris. Her spoken English is dated and genteel. "Thank you! Thank you so much !" she cried, both breathily and breathlessly, after arriving on stage via the stalls. "And thank you especially to the security lady who held my hand!"

She's 50 and fabulous, in a silky white vest, slim-fit black jeans and white plimsolls. The flyposters for the concert showed only her mouth, with its celebrated gap between the front teeth - a perfect imperfection, like Monroe's birthmark. In Row P, we couldn't see that clearly (a video screen would not have gone amiss), but we fell for her all the same. To paraphrase a line of Leonard Cohen's, she inhabits a hall quite beautifully.

Her voice is limited, but it's her: charming, natural, ageless. Her show is a shrine to Serge Gainsbourg, her late ex-husband and collaborator. The 20-odd songs are all written by him. "I heard that for the last time," she said after the second number, "when Deneuve read it in a little graveyard where Serge is." Just Deneuve; just Serge; very Jane Birkin.

She may be creating an archetype here - the widow-divorcee, as devoted to the memory of her man as Queen Victoria or Yoko Ono. "Why did you leave him?" an interviewer asked recently. She hesitated and said it was because he'd sent her a song that was sweet and sexy but didn't ring true.

Whether the rest of the oeuvre rings true is hard to say unless you speak fluent French. But musically, too many of the songs are thin and shapeless, even in the hands of a sympathetic nine-piece band. The rock numbers stick in first gear; the trad French tunes are better, but lack the punch that the Piaf-ish arrangements lead you to expect. "Je t'aime (moi non plus)", the once-scandalous orgasm song, turns out to be Gainsbourg's musical masterpiece. It is also Birkin's only British hit. She does not include it in the show. So you wonder whether charm is enough, regretfully deciding that it isn't. Then the curtain call arrives, and the audience (18 to 65, well-heeled), who have been sitting demurely, suddenly let go. There's a standing ovation, and another, and a third. Birkin fields the acclaim expertly. She bows so deeply that when an admirer throws her a rose, it lands on her back. Still bowing, she reaches up, grasps it, kisses it, smiles, and tucks it into her bra.

Asked for a second encore, she agrees, "but you'll have to sing it". Unaccompanied, she starts singing "La Javanaise", Gainsbourg's second- best tune, a spare and stately ballad. After a couple of words she leaves each verse to the audience. Perhaps 500 people sing along, word-perfect, and mostly in tune. It's the sort of moment that music is for.