And through the crush comes the woman whose name is on the door and the invitations. She is a figure straight from a kabuki Morte D'Arthur - over six feet tall, dramatically swathed in black Issey Miyake pleats, she has streaked-blonde Lady of Shallot tresses and her dark green eyes seem to radiate light. It is easy to be overwhelmed by Rebecca Hossack. Many impressionable young men have looked at Ms Hossack bearing down on them and lost their hearts. Many others have simply done whatever she required of them - sponsored an event, handed over a gallery space, accommodated her travelling band of bushmen and Aboriginal artists. Among those who have fallen under her spell are Lord McAlpine, Mrs Thatcher's former sidekick, Thomas Keneally the Booker-winning novelist (who launched his book Flying Hero Class in the gallery). Lady Colin Campbell, the mischief-making royal gossip and Barry Humphries. It is oddly appropriate that she should be mates with Bazza, since Ms Hossack was, in 1994, given the title of "Cultural Officer" by the Australian High Commission, saddled with the task of raising the profile of the Australian arts, a job she held through three years of wine 'n' opera evenings and art shows. She thus became the real-life equivalent of Mr Humphries's disgraceful caricature of an Oz cultural attache, the priapic, saliva-drenched Sir Les Patterson. She ceased to hold the job after 1996, but remains the Queen Bee of Antipodean arts in London, constantly promoting the work of native Australian talents, winkling out home-grown geniuses from dusty wastelands with comical names.
"Robert Hughes [the Australian-born art critic of Time magazine] calls Aboriginal art the last great art movement of the 20th century," she says. "But there's an awful lot of commercial crap you have to get through to find the real thing, and it's not at all what you think." She thinks there are "only 10, 10 maximum, really great artists in this field" and she has, of course, met them all. She has, indeed, often set off across the Outback in order to discover them, like an explorer hunting the source of a magic river.
"When I last went back to Australia, for the first two weeks, I swear if you hadn't known that Sydney and Melbourne were down south, you'd have thought you were in a black country. All these extraordinary remote places, with no white faces except maybe the petrol pump attendant's. And in a place called Fitzroy Crossing, miles from anywhere, we discovered this great artist, Butcher Cherel. All around him were kids and old men eating mangoes with the juice dropping down their front, and he was there doing the most incredible work, unlike anything you've seen in your life." She is absolutely sure of her judgement, even among the pointilliste jungle of dots and spirals and whorls that characterise these works. "You recognise it when you see a real artist," she says. "It's like discovering Damien Hirst. But most people don't know and don't bother to find out. So poor Butcher gets lumped as just another Aboriginal artist."
La Hossack is sweetly maternal about her charges - about the artists she exhibits, black, white, Western, African or flown in from nowhere. She talks you through the stuff newly hung in her gallery for its 10th anniversary this month, pausing before a vision of fat, grown-up angels (one strongly resembling Terry Waite) by Heather Flockhart, one of Michael Clark's fleshily unsettling paintings of a wound, a spookily blue shroud glowing in a dark cave by the amazing David Moore. As she rhapsodises about their work, you register the fact that at least three of her (other) recent proteges have teetered on the edge of breakdowns and asylums. She is full of concern about their stability. Her confidence that their talents will be recognised and rewarded - preferably before they go mad - is rather touching.
She's quite competitive about her maternalism - a social diva exasperated by people who don't know how to treat those outside ordinary forms of civilisation.
"Germaine Greer was very funny. When we exhibited Clifford Possum [the implausibly named but hugely distinguished Outback artist whom Hossack introduced to the Queen] for the second time, I asked Germaine to open the show. I took her and Clifford out to dinner afterwards and she was showing him the menu and going, 'Now Clifford, I think this scampi looks very nice.' Clifford looked so embarrassed - I didn't want to say, 'Actually, he can't read'. She said, 'Fish for you, I think' and I know he always likes a steak. She obviously thought fish would be more, you know, natural. And she said, 'Clifford, you'd really like to come and just sit in my garden and be under the trees, wouldn't you?' and he goes, Nah, I wanna go and meet some kunga - that's woman - and go gambling on fruit machines. She didn't really understand that he was just a red-blooded male on holiday."
Of course, with some deracinated tribesmen, there aren't any rules that apply, even for an acute people-watcher such as Ms Hossack. For instance, when the Gallery mounted a exhibition of the work of African bushmen in 1993, a score of the artists were flown over to attend. It was the first sighting of genuine Kalahari bushmen in London since Queen Victoria's Diamond Jubilee a century earlier. "They were so lovely," Rebecca sighs. "What really seemed to impress them was the lack of thorns on the pavement. How they could walk from A to B without getting scratched." Where did she take them? "Kew Gardens. Some of the other visitors looked a bit surprised to see 20 tribesmen strolling about." There was, sadly, no chance to try out the fish/steak theory, because "what they most wanted was Kentucky Fried Chicken. And chocolate biscuits". Did they drink? "Well, they weren't strictly allowed to have alcohol, but that seemed so ridiculous, so we had champagne all the time." That sounds very likely. You wonder if the super-ebullient Ms Hossack called them "Darling!" as well.
Behind the social butterfly you can soon detect a lively intelligence and a firm moral sense. She has little time for a man whom you'd imagine would have been her soulmate, the traveller, collector and Aboriginal expert, the late Bruce Chatwin. "All that Songlines stuff is a sort of creation of Bruce's," she says. "He used to come in here a lot and sit at that table because, he said, he liked Australian smiles. I read his book and thought it was all wrong. I know a lady he writes about, the one in the Alice Springs art shop, who was very annoyed by the way Bruce just went in while they were having a personal meeting and wrote down everything they were saying. He just didn't get it. The Songlines is a heartless book."
Rebecca Hossack was born in Melbourne 42 years ago. Her father was a doctor, an autodidact and an art collector. "He bought all the early Australian painters. I remember, when I was little, my father coming home and saying, 'I've just bought a Clifton Pugh painting of an emu in front of Ayers Rock', and my mother crying because what she really wanted was a vacuum cleaner. These days, she's very pleased and proud that he did buy them, but then ... " Rebecca was one of four sisters with Jane Austen-ish accomplishments: "Embroidery. We were all great embroiderers. Every Wednesday night of my life, I had to go to embroidery class with her. If you ever need any done, just shout." Could she paint? "No I couldn't. And still can't. I can't actually do anything. Except embroider."
She was always destined for the legal profession, and studied law at Melbourne University. Then she was sent to London in 1981, to go to the Middle Temple. "And I hated it so much, doing six years of a law degree." You mean, you hated the ethos of British jurisprudence? "Naaaow," said Ms Hossack. "I just hated England. I love Australia so much, more than anything in the world. Every morning, I would sit up in bed reading travel brochures about Australia and gazing at the blue sky." She sighed, a melancholy expatriate exhalation. "I never planned to stay. I came over, lived in a hostel in Mecklenburgh Square with the cockroaches, and I was so unhappy." But soon she discovered a passion for art, and for collecting one-off objets trouves around the world, that has never left her. She hung out with artists and was asked to organise "The Last Wapping Show", a farewell to the largest artists' community in the world, when its members were forced to vacate their warehouses at Wapping Wall in East London. "We got the Riverboat people to sponsor us, and ferried all these people down from Chelsea, we got artists to paint 70ft banners on the warehouses. It was wonderful."
Fired by her brush with an art organisation, she thought of opening her own gallery. In 1987, she cycled past No 35 Windmill Street and saw a sign saying "Space to Rent. No Premium Required" - that is, no pounds 30,000 key money was needed just to get through the door. The Rebecca Hossack gallery started life in March, 1988. At the same time she re-encountered the man who became her husband, Matthew Sturgis, the writer, historian of decadence and biographer of Aubrey Beardsley. "I met him 10 years earlier at the Uni [Oxford, actually] when I went to visit a friend who was living with him. They had a dinner party and people were talking about anchorites. I'd never heard anyone talking about anchorites before." The etiolated and charmingly English Sturgis re-encountered the statuesque and vividly Oz Hossack when he walked past the gallery in 1988, as she was unloading its start-up contents from a taxi, and that was that.
In 1991 she opened a sister enterprise - the Rebecca Hossack Sculpture Garden at the church of St James's, Piccadilly, at the invitation of the incumbent, Donald Reeves. "He said, it was either going to be a gallery or a soup kitchen. He approached me first, then I got Rio Tinto to give us some money. The idea was to have a little green square, where you could go without ever knowing quite what you'd find, in the heart of London."
She can sound, to be frank, the tiniest bit of an old hippy when she rails against the state of the modern city high street, where "All you see are chain stores, franchise shops, even the cafes and coffee shops. You can't have an individual experience. So I try and make this gallery more individual, with flowers and tables out in the street."
If you were trying to characterise the gallery, as expressed in its paintings, you'd say it was split down the middle between non-Western tribal art and a lot of home-grown surrealism and Scottish abstraction. "There's only one criterion to get your work shown here," she says, "and that is an extreme individualism. If you're following your own drummer, if I find an originality of spirit, that's all I care about. So many British artists are getting incredibly conservative. They're so conscious of promotion, they document their work so beautifully, they spend more time on that than on making the work." She cries up all her A-team of eccentric geniuses. "I've a Buddhist Jewish artist called Eva Holzhandler, a grandmother who comes to tea and asks Matthew, 'You mean, you've never taken LSD', and has a totally eccentric vision of the world."
She is proud of them as discoveries, just as she's beguilingly proud of her possessions, picked up on travels around 30 countries. She collects things with a manically inclusive passion, filling the walls and windowsills and tables and ledges of her small flat with little things - shell dishes and voodoo sculptures, magic scrolls and Tibetan yak headcovers, Aboriginal message sticks and sacred logs, snakeskin purses and even a few British items, like the 18th-century Westmorland oak desk that lurks underneath a whole treasure trove of objects.
Amid all these riches, all these must-have impulse buys, I wondered if she had a favourite. It turns out to be a little Ethiopian egg-shaped thing. "It's made out of earth, literally picked up off the ground, and cooked in a dung fire by the last Falasha Jew in Ethiopia. When you open it, inside are two little figures in bed with their arms around each, but all they're made of is two vertical lines, two horizontal lines, which are their arms, and two dots for their eyes. Everything about the human condition is in there, about being together, two becoming one, becoming whole - and all she's used is an oval, two straight lines and two dots."
One could happily listen to Ms Hossack's variegated raptures all afternoon. She is someone for whom the world holds a thousand surprises every day, yet never contrives to be surprising enough. Therefore, she delights in making things happen - exhibitions, sculpture gardens, riverside events, reputations. You suspect she also makes quite a lot of cash from virtually cornering the market in the best ethnic art, as Western dealers (and hotels and restaurants with empty walls) have woken up to its fin de siecle appeal, but that seems only fair. As I left, she was busy making plans for her next trip to the Australian outback, specifically to the not-very-high profile Lizard Derby at Kunnamulla. "Australia's getting a bit too touristy and PC and self-aware," she grumbled, "but Kunnamulla is an absolute wasteland, a two-day ride from Brisbane, a one-street town with a pub and opal fields everywhere. They have this derby with big and small lizards, with goannas and frill-necked lizards, and they race them. This year I'm determined to win it." I looked at her. "No, really, John. I'm getting mine specially trained." Christ, I think she means it