IT is Sunday afternoon, shortly after lunch: steak pie, mashed tatties, something green and healthy and tasteless, followed by apple crumble and a solid dollop of custard. The wife, having dealt with the dishes, is stretched out on the sofa, mouth ajar, snoring lightly. The kids are in their rooms communing via the internet with other kids thousands of miles away. You have supplied yourself with a mug of tea, studied with disgust the TV schedules, and have turned to the Sunday papers in the hope of a good read. In these "blissful" circumstances, what is it that you want to read about? "Naturally," wrote George Orwell, in reply to his own question, "about a murder."

This was the scene - not so subtly doctored for 21st-century sensibilities - set in Orwell's famous essay on the decline of the English murder. But what kind of murder did we want to read about in the years before Hitler's bombs began to fall? Orwell's research into celebrated cases threw up a variety of common factors. Six of the real-life crimes he looked at included poisoning. Eight out of 10 of the criminals belonged to the middle-class. Sex was never far away as a motive. In several cases respectability - "the desire to gain a secure position in life, not to forfeit one's social position by some scandal such as divorce" - was a prime reason for committing murder. Money, too, was frequently at the root of the evil act, but not necessarily large amounts of it. And more often than not curious neighbours and preposterous coincidence led to the unmasking of the killer, who was probably known or related to his victim.

All of the murders looked at by Orwell occurred between 1850 and 1925, what he dubbed "our Elizabethan period". These were murders that had stood the test of time, involving legendary killers such as Jack the Ripper and Dr Crippen. Orwell was convinced that this era was over and that more mundane murder was the order of the day.

Like Orwell, PD James, also known as Baroness James of Holland Park - where she lives when in London and where we are taking afternoon tea in an archetypal English sitting room familiar to devotees of Agatha Christie - thinks murder has become more commonplace, less exotic, less puzzling, less extraordinary. Has it, then, been devalued in the hierarchy of human venality? In some respects it has, she suggests. "People don't cover up murders now." Nor, she adds, with the removal of the death sentence, does it strike the same fear in the hearts of those considering committing it. "I think the average length of sentence for murder is about 10 years, isn't it? That isn't life."

It is said not in a "hang and flog 'em" tone, but sweetly and with a hint of sadness. James is 88, and during this year she has written The Private Patient, a very English detective story of the type whose demise Orwell mourned. It features Commander Adam Dalgliesh, who has been prominent in James's work since 1962, when her debut novel, Cover Her Face, appeared. The Private Patient marks his 14th, and perhaps last, appearance in print. He is a man, one suspects, very much in his creator's mould. He's no maverick, as fictional detectives invariably and predictably are. He doesn't bend rules or work against the system. He doesn't drink to excess and his personal life is respectable. When off duty he writes poetry, published, like her novels, by Faber.

"He is a man who is absolutely punctilious and has no sympathy at all with colleagues who go over the line because he thinks it is very dangerous; because of course it is very dangerous; because of course it can lead to injustices. But I think he is well aware of implications of the law - as we all are - and I think in this book A Private Patient there's a suggestion that he's wondering if he's had enough."

Has he? "I think he may have had enough," James says, considering for a moment the implication of that remark on herself and her legion of loyal readers. "I can't see myself giving up writing but whether I'll write another detective story, it's rather too early to say."

Reviewing her long career, has she achieved what she set out to do? "I think as far as detective fiction is concerned, yes I have. Whether it would have been fascinating to have written a good play, I don't know. I don't feel honestly that I've got any great regrets. I don't feel there's something I should have done. I like the idea of a play but I'm not sure I'm the kind of writer who likes the co-operative aspects of it: being at the mercy of a director or a producer, having to give way to the foibles of your star actor. What I like about the novel is it's your novel. It might have defects but on the whole nobody interferes with it. And I think I like that. I like the sense of being in control."

A Private Patient is a quintessential example of James's art, the kind of murder mystery of which Orwell would surely have approved. Set in an enclosed community, a clinic in Dorset which specialises in plastic surgery, its cast includes a surgeon, an investigative journalist and a shoal of red herrings. Dalgliesh becomes involved for silly reasons, which inflames his sense of bureaucratic propriety. Moreover, he is on the brink of marriage, another clue to James's desire to have done with him. Like a mother seeing one of her children off to university, she is ambivalent about losing him but knows that all good things must eventually come to an end. In any case, Dalgliesh, unlike her, has not aged as he naturally would have. He is still young while she - mentally youthful but physically restricted - is nudging towards 90.

Her detective, surgeon and journalist share several characteristics. All, for instance, are exceptionally good at their jobs. All, too, are reserved when it comes to their private lives. The same might be said of James, who has always in interviews refrained from giving too much away about herself and her personal history. In part, this is undoubtedly from a desire not to pander to the prurient. But it is also her nature, a mark of her Englishness.

In Time To Be Earnest, the diary she kept during her 77th year, she wrote: "There is much that I remember but which is painful to dwell upon. I see no need to write about these things. They are over and must be accepted, made sense of and forgiven, afforded no more than their proper place in a long life in which I have always known that happiness is a gift, not a right. And there are matters over which memory has exercised its self-defensive censorship. Like dangerous and unpredictable beasts they lie curled in the pit of the subconscious. This seems a merciful dispensation; I have no intention of lying on a psychiatrist's couch in an attempt to hear their waking growls."

If that sounds like a warning shot across the bows of the intemperately curious it probably is. In person, though, James does not so much ignore inquiries of a personal nature as deflect them. At one point in our conversation I remark that it is a pity the investigative journalist had to die when another client of the clinic would have been less missed. "We've all got people we wouldn't grieve to see the last of," says James. Who she would nominate she doesn't say. In her fiction, however, she knows from the outset who will live and who won't, "who's going to be killed and where and when and why and by whom, and who the suspects will be." Detective fiction, she adds, is a "reassuring" form of popular literature, and can be a reassuring form to write. "In other words, I've chosen the form that suits me best. And I think that I've achieved probably as much as I am personally capable of achieving with it."

Such a modest and pragmatic assessment of her talent is typical of James. She began writing, she recalls, when she was in her mid-30s, a late start for someone who knew from her childhood that she wanted to be a novelist. Her father worked in the Inland Revenue and earned enough to buy a house, raise and educate three children, two of them at private schools, and allow his wife to remain at home. "Life now," she says, "is tougher." Her childhood was by and large happy, but blighted by the knowledge that her parents were locked in an unsatisfying marriage. Her mother, she has recalled, was sentimental, warm-hearted, vivacious, with a love of music but lacking in intelligence. In contrast, her father was "intelligent, reserved, sarcastic, deeply distrustful of sentimentality, fastidious and with little ability to show affection". In that regard he is a dead ringer for Dalgliesh. Today the likelihood is that they would have gone their separate ways. In the years between the wars, however, there was more chance of them murdering one another than divorcing.

James's own marriage was unhappy, but for very different reasons. Her husband, Connor, and she were almost exact contemporaries. They met while she was working at a theatre in Cambridge. He was a medical student who returned from service overseas suffering from mental illness. The prognosis was not hopeful and his later years were spent largely in psychiatric hospitals, in which, she says, he was never unhappy, having been well prepared by his education at a minor public school and in the army. Talking about him is clearly painful to her and she alludes to him only in passing. In her diary, she wrote that she had never found "anyone else with whom she wanted to spend the rest of her life". She would visit him in hospital, where he'd decided to call himself Ted. "For some time," she recalled, "he worked in the library but also captained the soccer team. I don't know whether any games were played away, but those on home ground had their moments of eccentricity. Connor was not pleased when, during one game, the goalkeeper began hearing his voices and stood immobile, eyes raised to heaven, while the ball whizzed past him into goal."

James started writing when she was working at Paddington Hospital, writing for an hour every morning before she began her commute. "The work was hindered by family emergencies, by pressure of my job and by the need to spend some evenings at the City of London College in Moorgate, studying for the qualification in hospital administration which I hoped might eventually result in a job sufficiently well-paid to support my family."

Though her first novel was greeted with a cluster of appreciative reviews, there was no question of her giving up the day job. Indeed, her parallel lives continued until her retirement from the Criminal Law Department in 1979, by when her coronation as the next Queen of Crime following Agatha Christie's reign was assured.

It is a title she still holds. Like her hero Dalgliesh she abides by the rules of the genre, never springing surprises on readers, offering just enough clues to make detectives of us all. In the end, order is restored and life goes on. There is respect for law and order and belief in justice. Her milieu is not mean streets and such violence as there is never glamorised or graphic. En route to the denouement we are offered paeans to London and its great river and to the seductiveness of the English countryside and its ageless architecture. Never ever, though, does PD James allow us to forget that her business is murder. So has murder really been devalued? Not in her mind it hasn't. "Though there is a different attitude to it, it is still regarded by most people as a unique crime. It's commonplace to say a fate worse than death' but most of us feel there isn't."

The Private Patient is published by Faber at £18.99