"Until Monday I was seriously worried about what I could possibly do next," says St John, lighting the end of a roll-up cigarette and exhaling smoke into the gloom of her top-floor sitting-room. "I mean, my sales were so pathetic that with the best will in the world no one would want to take me on. I thought I could be a check-out girl, but I'm probably too old. However, I did really talk myself into being a tea-lady." Her bird-like frame shakes with laughter when she recalls her desperate contingency plan.
But overnight everything has changed, and the Australian-born St John is a subject of media curiosity and attention.
Her sharp eyes are framed by an auburn bob. Her pale complexion looks as thin and brittle as a sheet of paper. "All this changes one's perception. I don't know what it will be like to return to real life."
It has to be said that real life, to judge by her flat, has hardly been moving at a furious pace. Indeed, the decor seems to have stood still: an antique clock in one corner has stopped at a quarter to eight, while the scattering of white Sixties furniture must have faded to nicotine yellow long ago, and the white net curtains are yellowing too. There is a touch of Miss Havisham about it all.
People assume that St John, 55, must be a recluse because she has such a low profile. "I'm a recluse to the extent that all this is a total novelty for me," she says. Her third novel, The Essence of the Thing, examines the on-off relationship between two middle-class Notting Hill thirtysomethings, Nicola and Jonathan. The plot unfolds after Nicola nips out to buy cigarettes and returns to find that Jonathan wants to leave her.
Critics have already sharpened their knives, describing her interest in everyday minutiae as "the last word in banality", displaying "all the depths of stick figures on a road sign".
St John is undaunted. "I mean to write about something beyond those details," she says. They are just there to lead you to another place. That's just the surface. I know some people can't see that, they see some dreary little account of a couple breaking up, but what the hell."
Her book is an astute observation of English middle-class manners. She is peculiarly affectionate about our class system. "My feeling about the English is, they love to ham it up. They act their role to the hilt; Cockneys do and so do the upper classes. I think it's a great and wonderful spectacle after coming from such a monotone place."
St John is as middle-class as they come; speaking just like one of her well-bred characters, only the faintest Australian twang remains. Born in Sydney, the daughter of a barrister, she was educated at boarding school and Sydney University. When she was 12 her mother died which, she says matter-of-factly, "obviously changed everything".
After university she moved to the United States with her husband, who had been a fellow student, and worked in a bookshop. The marriage was over by the time she reached England two years later. "You can guess what it was distracted him," she says knowingly. "A woman, of course. I just wish it had been someone nicer."
She has no children and has never remarried. Is she too choosy? "Women I know actually say, `Something is better than nothing', but I'm only interested in `really great'. Being single can often be inconvenient, though - it's better when you grow old."
When she first came to London, St John supported herself through part- time jobs and worked in a left-wing bookshop. "It was full of earnest,dedicated socialists," she recalls. "I didn't believe a word they said - I knew that life wasn't that simple."
She turned to writing, purely to "try and earn a bob or two. The thing that stopped me for so many years was the realisation that I couldn't be that good. Then it percolated through that I could be as good at least as some of the people who were getting away with it."
And maybe a good deal better. Meanwhile, St John sits around, slightly bemused, while journalists trail through her flat, probing an obscure history. She admits that she is "egotistical", but remains realistic. There is no false modesty when she says: "I think Ladbroke's odds are right. Of course I'm not going to win the Booker Prize, I just know that," and stubs out the last of her roll-ups with conviction.