Then the door crashes open and a ragged schoolgirl sweeps in. Her hair is long, her foot is light and her eyes are wild - la belle dame sans merci in a shapeless oatmeal sweater, jeans and trainers. Regarding the suite without actually looking at anybody in it (a useful technique possibly acquired at Miss Ironside's legendary Academy in Kensington, which she attended in the Sixties) she explains that she is late because she was "stuck in the tunnel", ie the Eurotunnel - a perfect metaphor for the half-way Franglais celebrity Jane Birkin has become.
She is over here to publicise her greatest hits album (out 31 March) and her forthcoming concert at the Festival Hall on 15 April. At the mention of both these enterprises, you can detect a faint collective raising of an eyebrow from British culture vultures. Greatest hits? From a woman whose only British hit was the notorious "Je T'Aime... Moi Non Plus" in 1969, a duet with Birkin's lover Serge Gainsbourg featuring a lot of orgasmic breathing and, if I may so express it, an irresistible organ riff? The Royal Festival Hall for a singer with the vocal range of a sparrow and the emotional pitch of a nervous 14-year-old? Hah! You bring up Ms Birkin's 50-movie film career, much of it in the hands of major art-house directors such as Bernard Tavernier, Agnes Varda and Bernard Blier, but these disobliging commentators would express the same disbelief. ("Oh, her films? She got her kit off in Blow-Up, didn't she? And kept it on for Death on the Nile..."). It's a tough break, but Ms Birkin has never really managed to convince British audiences of her multiple talents. Francoise Hardy, the singer and a close friend, never had this problem, though she never had a Number One single in England. Huppert and Adjani may be directed by the same directors but they're not treated as if they're just lucky (and they never appeared on the stage of the National Theatre, as Birkin did, playing Andromache in The Trojan Women in 1995 to excellent reviews).
Ms Birkin has been a hardworking actress, singer and all-round French media tarte for more than 30 years, but her reputation stays firmly lodged on just one side of the Channel. There, she is feted, adored, lionised, stopped in the street and is never off the telly, which once featured a whole "Birkin night". She is the embodiment, as far as the French are concerned, of English charm. And if one has reservations about her acting, there's no doubt about the charm.
"Yes, I'm staying with my mother, who is lodging in a little - how do you say? - flat because her house burned down. No, she's taking it extremely well, actually. I worried that she might be depressed that everything she owned had gone up in smoke. She was eating fish and chips with the firemen, glad that it was not her fault, and saying that she felt 'lighter'. So I followed Ma's example and sold my house in Paris and moved into a wee apartment in St Germain. Just decided to get out of it all." Did it coincide with hitting 50? "No, it coincided with discovering somebody I thought was perfectly divine and finding an apartment opposite where he lives." Is it love? "Certainement. And now you must ask me a proper question."
She really talks like that - en grand vitesse - full of little verbal slaps and tickles and Frenchified syntactical manglings. She is far too charming to seem to monopolise the conversation; but she certainly does colonise it, filling every sonic cavity with passionate reminiscence. Her voice is pitched at a querulous, insistent but not unmusical high note, and her delivery weirdly time-warped into that Fifties actressy mode favoured by Joan Greenwood and Audrey Hepburn. She plays constantly with her chestnut hair, winding it round her head and tousling it coquettishly over her eyes - which do at last occasionally look at mine, with a disconcertingly beautiful alien glow at their centre.
I said the weekend papers had been full of how much the French supposedly love le style Anglais, as embodied by Alexander McQueen, John Galliano, Terence Conran and Stella Tennant. Did she notice any new wash of admiration for the Brits? "Oh, but they've always had it. And I've always been longing for it to be vice versa. When I arrived there 30 years ago, the greatest thing was to be English, to have an English accent, English teeth... the music was English, the fashion was English." But weren't we equally keen on French things lately, whether it's the film of Ridicule, the paintings of Braque at the RA, the intellectual cross-fertilisation inspected by Julian Barnes in Cross Channel... "It's happening a leetle," she said. "It could hardly have got much worse, could it?
The name of Serge Gainsbourg crops up often to illustrate a point about culture. Stick around - Mr Gainsbourg's name is to recur throughout the ensuing hour. Although he died at 62 of a heart attack, in 1991, Jane Birkin still carries a torch the size of the Olympic flame for her former beloved. He was 17 years her senior, and they split up in 1983, but she describes him and his work as if they were newly newlyweds: "So funny, like Woody Allen or Mel Brooks, so outrageous, so sad and terribly romantic, one of the most sentimental people. And he was someone who made his own fashion, with his little bare feet and his little white shoes." It's all a tiny bit private. She will talk about his public life at the tiniest provocation, pointing out that "the man whose name springs to the lips, when these young bands are talking about French singers, is Serge's" and "Children in schools are now being taught Serge's work. He's known to be the greatest poet... It's no exaggeration to say they consider him as great as Apollinaire." My ignorance of French literature means I have no way of confirming this; but until it becomes true, Ms Birkin will be an unstoppable one-woman lobbying campaign.
One curious result of this obsessiveness is that the Birkin song repertoire has hit an impasse; she displays no interest in singing songs by anyone but Gainsbourg. "Phonogram brought out two records of mine last year," she says with a smile, "but they said I couldn't go on singing Serge's songs because people had already bought them three times over. They said, 'You really must find somebody else's stuff to sing'."
I said it seemed quite a good idea - maybe some selections from Evita would suit her extraordinarily teeny-weeny voice. "Yes, but I didn't want to," said Birkin, verbally stamping her foot. "The time wasn't ready. I still hadn't done enough with what Serge had given me, which were probably his most beautiful songs. So I pinched other songs that Serge had written for other people - for Adjani, for Deneuve, the ones he wrote before he was 30 when he was in cabaret on the rive gauche." She also ("Oh, the cunning of it!") got a new artistic director to orchestrate the selections for unusual instruments - for slide guitar, for harp - and a further collaborator to work on the stage show with tubular bells and thrumming cellos and was a wild success. One reason was some advice from the director to the melancholic chanteuse: "He said, 'It would help if you smiled, if you wouldn't mind'. He said, 'It does make a great difference to the audience, otherwise they start to feel terribly responsible for you'. So I tried it and it worked a charm."
She was born in December 1946 and brought up in London. Her father was a war hero, a Navy navigator who helped French Resistance on the run. "Ma was waiting for him in London, not understanding why he wasn't home for their first Christmas together. It was because he did about 40 missions and never made a mistake." Ms Birkin's eyes shine proudly when she talks of her dad, who died in the same devastating week that Serge Gainsbourg checked out. "He was just a very unexpected father. We marched together on Stop the Death Penalty campaigns. He wasn't just a hero, but a social hero, too." Her mother was Judy Campbell, a singer and stage actress, and the first interpreter of "A Nightingale Sang in Berkeley Square". Her father sang "My Favourite Things" from The Sound of Music, "rather sweetly, in a tiny high voice that used to make us die laughing" and both parents loved musicals, especially Kismet, South Pacific and Guys and Dolls. Jane herself has wondered about a possible career on the musical stage but... "It's a great sadness for me that France is a country I adore above all others, but what a pity they don't like musicals. They get it all mixed up with operetta, which is of course nothing to do with it. Sometimes I try to sell a producer the idea of a musical - I thought I'd got to the age at which to play the governess in The King and I - and the more I waded through the plot, the more it was as if I'd brought something out of a tomb."
A precocious teenager, a King's Road debutante, she was taken up by Binkie Beaumont, the legendary producer and friend of Noel Coward (who used to write songs for her mother) and found herself on stage at 17 playing a deaf mute (very much against type) in Graham Greene's Carving a Stone with Ralph Richardson. After appearing in the girls' school-brothel comedy musical The Passion Flower Hotel, she didn't return to the English stage for 30 years. Instead, she was romanced by John Barry, composer of a million film scores (and the James Bond theme) whom she married at 18, had a child by (Kate) and divorced at amazing speed. Visiting Paris for an audition, the non-French-speaking Birkin encountered the scruffy, exophthalmic Bohemian Gainsbourg and that was that. They stayed together, the decadent white Russian composer and his classy English moll, for 13 years, epatant les bourgeois and becoming the nation's most cooed-over couple. They had fantastic rows in public.
When they split up, Birkin starred in Tavernier's These Foolish Things with Dirk Bogarde, who remains a friend, in La Belle Noiseuse with Michel Piccoli - and in 1980, playing an unglamorous frump in La Fille Prodigue, she fell for the director, Jacques Doillon, with whom she had a third child, Lou, who lives with her still.
Now she is 50, a fixture on French television (she says she is on some channel or other once or twice a week), she remains clearly a little hurt that her real homeland never clasped her to its fickle bosom. "In France, you're family. If you're on television, you're part of the family. They ask, 'How's Charlotte? How's your dog?'. But of course I've never done anything very public in England. Once the concert is behind her, and her next stage appearance (in David Hare's play, Skylight, in Paris) she plans an unusual escape. "At the end of the year, I shall wrote another movie, and then start my wanderings. I want to have that year English talk of, the "gap year", and go to Africa and Vietnam and South America. There are these tiny pockets of French culture all round the world, and the people who stick up for French culture tend to be rather mad, as if they're sticking up for animals who're becoming extinct. And they all watch Canal- Plus [the French satellite TV station]. And you find that you're better known in Burkina-Faso than in England."
What she's best at is being herself, a talent she has parlayed to the whole of France by seeming to embody the whole of England. If only it would work in her own backyard. "All the cultural attaches in these strange parts of the world," she said ruefully, "they have the bright idea of asking you over, 'to defend the French language'. It's somewhat to my surprise to find myself asked to defend the French language, I who have deformed it for so long..." Ah, charmante.