Jane Birkin’s schedule for today. Instruct sexy-voiced male assistant to phone The Sunday Times and remind them that ze alloted hour for telephone conversation is in French time, not English. Call on residual English good manners while explaining, in words of one syllable, why Serge Gainsbourg is revered throughout the Francophone world. Break off conversation to change travel plans in fluent but still boarding-school-accent French. Record short pieces for French and Canadian television. Then get to venue outside hometown, Paris, to peform Arabesque, a show of Serge Gainsbourg songs rearranged by Arab musicians, for the 99th consecutive night. “I have not,” she says without the slightest hint that she is about to collapse in a heap, “had a day off since January.”

In Britain, Birkin’s name is forever synonymous with the breathy Je t’aime, moi non plus, recorded with Gainsbourg in 1969 and lying somewhere between a Black Lace novelty record and the soundtrack to some dated, pseudo-arty porn. Banned by the BBC, it immediately reached number one.

Film buffs of a certain age also retain happy memories of her naked scenes, which included the first screen glimpse of pubic hair, in Blow Up, while for fashion-literate women, Birkin’s name brings instantly to mind the Hermès handbag based on a sketch she did for M Dumas of the French fashion house in the 1970s. It is still an international cult purchase and one of the Hermès bestsellers.

Birkin and Gainsbourg summed up the age appearing, from the outside, to be one couple whose 1960s really did swing: he was the louche ladies’ man, a 40-year-old actor, singer and songwriter who had already wooed and won Bardot and Bambou. She was the English public school girl who, by the age of 20, had become an art-house star; as wide-eyed and waiflike as his previous conquests were pneumatic.

They met in 1968 when she was cast opposite him in Slogan. He claimed, rather improbably given his sexual CV, that he was attracted to her because he was scared of breasts.

They were the bohemian Posh and Becks of their day, pursued by the paparazzi, living a lush life of long lunches and parties, her in haute couture, him in the diamond she bought him to wear around his neck. She accepts little credit for Gainsbourg, who was already a fully formed reprobate when their relationship started, but does claim that she persuaded him to grow the three- day stubble which became his trademark.

Gainsbourg died in 1991. Birkin had ended their relationship eight years before but they kept a room in each other’s home; he continued to write songs for her and she has continued singing them. The Arab influence is a more recent development although it is one she is sure he would support. Arabesque began its life when she was invited to the Avignon festival, which she compares to the Edinburgh Festival, in 1999.

“I didn’t quite know what to do,” she recalls, her cut-glass voice high-pitched, perfectly modulated and hard to reconcile with the panting temptress of Je t’aime. “I didn’t have a cute one-woman show with a few jokes and a bit of Cole Porter. I was about to say no when my artistic director, Philippe Lerichomme, who is the man who told Serge to get off and do reggae, said you’d better do something exciting, even if it’s dangerous. You’ve got to knock them out a bit.” What he had in mind was a collaboration with the Algerian violinist Djamel Benyelles, a classically trained musician who had also been influential in bringing North African tunes to world music audiences in France and abroad. Lerichomme had already asked Benyelles to have a go at a Gainsbourg song and invited Birkin along to have a listen.

“It was Elisa, a very light song Serge had written for Zizi Jeanmaire about 25 years ago,” she recalls. “I heard this extraordinary orchestration, the indulgence in melancholia was very like Serge and then suddenly it came pivoting into something that was danceable, then coming back with the sadness. I was so overwhelmed that I realised immediately that I was onto something like a treasure.”

Since then, Birkin has performed Arabesque all over France. She has taken the show to Algeria several times, where the popularity of French television, delivered via satellite, mean that Gainsbourg’s songs are as ubiquitous as they are in France, and Birkin is a big star. “At the airport they said, ‘You’re home.’ Nobody’s ever said that before, not even in my own land. North Africa has become a kind of home for me.”

Back in Paris, she bumped into the director of the Odeon, the French national theatre. “He said that he had a hole in his programme, had I anything that would fill up this hole. I said I do have something; we made posters ourselves and put them in the Metro and it was absolutely chock-a-block, completely packed. It was one of the most exciting moments of my life. To go to the national theatre with Djamel, with the show that was only supposed to be one night in 1999, was just wonderful.”

Among the crowds at the Odeon was the man who is now Birkin’s tour manager and who is responsible for her current relentless schedule. Her exact age has been lost in the swirling Gauloise smoke of time but she is a grandmother in her mid-fifties tackling a tour that would challenge S Club Juniors. Apart from the stamina required to sing emotionally demanding music at the top of her vocal register every evening, if she is performing outside the French-speaking world Birkin faces the additional challenge of putting the past into perspective once a night.

“In Germany they know me a little bit but it’s always with references to Je t’aime. In France, Serge is considered as Apollinaire. The cheeky side of his character and his stunts on French television and all that were a plus, it was the eternal adolescent that Serge was, but in fact he’s taken very seriously. They learn his works at school.”

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