Twice impossible; one, because of the subject matter - conscious self-knowledge about things so painful they are unbearable; and, two, finding a method that articulates inarticulateness.

St Aubyn sat down to write the first Patrick Melrose novel, Never Mind, in 1989. There are stories of him so distressed and sweating that he wrote wrapped only in a towel. But he had found his method. Bad News, Mother's Milk and Some Hope, arriving in a series of abrasive and luminous mosaics, individual panels of definitive moments in Melrose's life, appeared consecutively in 1992, 1994 and 2006. At Last, published in 2012 and covering the two hours of Melrose's mother's funeral, concludes the quintet.

At the end of February, St Aubyn will visit Australia for the first time. Right now, the writer says in his warm, elegant drawl (''Paar-trick'' is his pronunciation of Patrick), he is ''sitting in a chair in my drawing-room looking out into a leafless tree on a grim London morning''. Interviews make him anxious and he has been up since 4am, worrying.

It isn't that he hasn't wanted to come to Australia before for a holiday, but the distance is great and he doesn't like leaving his family. When his friend Alan Hollinghurst was here last year, he wondered if St Aubyn might not be perceived as ''too much of a toff'' by the egalitarian but notoriously touchy Australians. St Aubyn is aristocratic and his novels deal with the fabulously rich upper crust who have houses all over the world and nobody ever seems to have a killer job, but he reserves a particular venom for the snobbishness and the fatuousness of those who feel entitled to the world by the solitary fact of birth.

St Aubyn's work has its source in subjectivity, but by spectacular inventiveness and discipline he has transformed the raw material into the unique art that maintains a meticulous balance between the hysterical and the astringent. But it is unhappy and common knowledge that St Aubyn has had rather more raw material than most writers care to contemplate.

Scion of an aristocratic Cornish family, St Aubyn was five when he was raped by his sadistic father, who told him not to tell his mother or he would be killed. The attacks stopped a few years later when the young boy made his own heroic stand. St Aubyn went on to become a heroin addict. As critic James Wood tartly said in The New Yorker, ''he has spent much of his adult life trying to kill himself with drugs and booze''.

He has long stopped trying to kill - or was it console? - himself. ''I don't know that I want to talk about these things but … I was shockingly young when I first started [heroin] and it went on 10 years, or 12. Bad News is a portrait. Compressed, of course. It's self-explanatory: people take drugs because they can't bear their own thoughts and feelings. The only thing powerful enough to displace powerful thoughts and feelings are extreme sensations. There you have it.'' That he managed to stick through school and complete a degree at Oxford when his life was breaking to pieces suggests something heroic in his nature.

He emphasises, politely, that his novels are precisely that, novels. Fictions: created, filtered, invented, wrought entities that make use of the shocking raw source and result in transformation not confession.

''I feel it's important to realise how different this is from memoir. Patrick Melrose is an alter ego, his parents are a portrait of my parents but so much is invented, so much is conflated. There's the narrator, the writer, the character, and even the writer is only one aspect of self. And there are all these gaps. On it goes.''

Confessional works do not interest him, but he sees the Melrose quintet as ''a truth-telling project''. The project opens with just a day and a half in the life of five-year-old Melrose and concludes with the character at 50.

It's a commonplace that people write to ''get better'', but St Aubyn finds it ''strange and exclusive to say the way to get better is to spend 20 years writing a book''. For him it was the opposite. ''Each book was extremely difficult and disturbing in a different way and in the writing [of them] it wasn't immediately obvious that it was going to be a liberating project. But I do think there is something fundamentally salutary in taking something painful or ugly or unbearable and even unthinkable and making out of it something that is a source of pleasure. I'm very lucky. It's not that obvious while you're doing it, that's all I'm saying. After At Last I found some peace of mind, but along the way I found it disturbing; and then when I finished felt obligated to write another one.''

About the peace of mind he says: ''How much it is to do with books or other people and their extraordinary kindness I don't know.''

St Aubyn's early work has echoes of the matchless comedian Peter Cook, whom St Aubyn met. ''He was such a fan of my first two books. I used to love him when I was a teenager. Incredibly brilliant and funny.'' Cook's intensely British comedy is perhaps as much an influence on St Aubyn as those most often mentioned: Waugh and Wilde. St Aubyn is as hilarious as both and more savage than either.

But it is Proust, whose genius was in finding a new way to transform subjective experience into a radiant art, who provides deeper parallels. When St Aubyn was 24, he rented a cottage in the English countryside to escape the turbulence of his London life. He intended to live an isolated life and to write. Instead, he read Proust's A la recherche du temps perdu.

''Proust is a hero of mine. I read A la recherche in one go, and I'm a very slow reader. It had an astonishing impact, reading it on my own and being my main company. I think Proust is the most intelligent person to ever have written a novel. A la recherche gave me an attraction to dealing with time and trying to write an accumulative novel.''

They have much in common, especially their use of mimicry and ravishing metaphor, and St Aubyn has also mastered the movement of time in a very different way. Where Proust digresses and elaborates and then embroiders upon digression, St Aubyn is obsessed with compression and precision. Where Proust is vast, St Aubyn is brief. In speed and compression, Racine is his master, not Proust. Jane Austen could have been his tutor, perhaps, in regulated hatred.

At Last is compressed into two hours, although it returns many of the characters from the previous novels. Most are vile, but the vilest are often the funniest as they reveal themselves and strike terror into the reader's heart with the truth of the human condition and the discomforting familiarity with their weaknesses.

St Aubyn believes he has finally finished detailing Melrose's life. Although, as the character's charming and serious son Thomas observes, the reason for minds is that ''they can be changed''. Maybe some time Melrose could be allowed something like happiness? ''Well, the attractive thing about the subject of happiness is that it is notoriously difficult to write,'' St Aubyn says. But he feels that, in a future novel, Melrose would have to go beyond preoccupation with his parents and his past.

At Last represents the end of 20 years work for St Aubyn. Patrick Melrose, the heroic hysteric, has arrived at a state of self-knowledge in which he sees that some ideas he had about his past and his parents are perhaps wrong. He can contemplate the ''idea of a voluntary life'' rather than one of reaction. The words ''goodness'' and ''kindness'' hover and not in any ironic sense. With this shift he can perceive the core of vulnerability and helpless inarticulateness in himself, and others.

The most addictive consolation for Melrose is the English addiction to ''irony'' and it is this, which distances other people, that is going to be the most difficult to renounce. St Aubyn refuses to comment on content. ''It would diminish 20 years of work to paraphrase. It can sound like a therapeutic sound bite, which it is not. It's an experience that the reader has after a long journey.''

His readers? He has no idea who they are but is eager to engage. This, in itself, is amazing because once ''the idea of meeting the public publicly was the most terrifying thing I could imagine''.

His terror had a miraculous cure. After Mother's Milk came out and appeared on the Booker shortlist, his publicity team soothed their reclusive author into a public appearance at Oxford. There he found himself confronting an audience who might have been the darlings of his worst paranoia. But he survived.

His Australian readers have reason to be grateful to that Oxford audience - they have in store an evening with the best prose stylist writing in English.

At Last is published by Picador, $19.99.