THE SEEDS OF TREASON By Ted Allbeury Mysterious Press. 262 pp. $15.95

THE BRITISH spy novel has its conventions, just as rigid as the stately home murder mystery. Unlike Ian Fleming's Bond books, which bore no relationship to the known world, most espionage novels of the realistic school are set in an atmosphere of constant gloom, relieved only by occasional violence. The hero leads a life as seedy and threadbare as his raincoat. His wife is usually unfaithful, and in one recent novel, is actually a Soviet spy.

The plot is so impenetrable that the Enigma machine itself would need a week to figure it out. At the end of most thrillers you learn who did it; on finishing, say, a le Carre', you may decide that you're not smart enough to deserve to be told. The conventions were easy to establish, since, because of obsessional British government secrecy, few people had the faintest idea of what the spy's job was actually like.

Then this year along came Peter Wright's book Spycatcher, the nonfiction memoirs of an MI5 officer. Spycatcher wasn't just realistic; it was real. In it I found something more intriguing than the well-publicized revelation that senior intelligence officers had pondered trying to bring down a British prime minister, or the suggestion that the head of MI5 might have been a Soviet agent. It was the description of day-to-day life in the service, which, if Wright is correct, is thick with incompetence, snobbery, feather-bedding, lack of money, jealousy, crass stupidity, pointless rivalries and damaging loyalties.

In short, the Secret Service turns out to be similar to every office most of us have worked in. Now that more than a quarter million copies of Spycatcher have been sold in hardback, in the United States alone, I doubt if things can ever be the same again for readers or writers of the genre. It's as if a Martian were to show up at a convention of science fiction writers and say, "Look, I hate to disillusion you guys, but really Mars is a lot like Nebraska."

So maybe The Seeds of Treason will be among the last of its kind. It ought to be pretty realistic, since during World War II the author was in Army Intelligence (in Britain as here, often thought of as an oxymoron), and according to his publishers was the model for Len Deighton's lugubrious hero Harry Palmer.

THE PLOT is about four people who for various reasons become traitors. Jan Massey, the hero, is half Polish, half English, which accounts for his mood swings. He is trapped into doing a job for the Russians through his love for a KGB officer's wife. She is, of course, fabulously beautiful, though from my limited experience of femmes fatales, they are often very ordinary.

A brilliant American mathematician on the National Security Council unwittingly marries a Soviet spy. Arthur Johnson, a sordid British army private hands over secrets for money and sex, and another small-time Brit goes over because he hates authority.

The book rattles along briskly, and the author knits his different strands together skillfully. The trouble is, we know now that life isn't like this. For one thing, these boys seem to have no trouble cracking Soviet codes, though Wright tells us that the West very rarely can. And the spies are all decent fellows, including some of the Russians. Massey may have helped the KGB, but he did it because he had the decency to betray his country rather than his lover. The American mathematician is a reformed rich brat who would rather go to jail than denounce the only woman he ever loved.

The rest are mainly honorable men, doing a difficult job under trying conditions. So where are the vengeful office rivalries, the alcoholics, the petty office tyrants, the promising youngsters passed over because they went to the wrong school? Where's the reality? That's the trouble with truth: it isn't just stranger than fiction, but a whole lot sleazier too.

But the book is fun and the plot is curiously easy to follow. It also has the greatest virtue any thriller can have, which is to avoid wasting the reader's time with long, unwanted backgrounds and atmospherics -- something of which le Carre' has recently been guilty.

My problem is with some of the dialogue. Americans and Brits don't just have a slightly different vocabulary, but speak in different rhythms end use different sentence constructions. Allbeury's Yanks talk like Brits. And his romantic scenes are as leaden as week-old suet pudding. "She smiled. 'Make love to me, Jimbo,' " is an instance. I suspect that someone has secretly marketed a computer program to write sexy dialogue for authors who are easily embarrassed. It writes the scenes while they make cups of tea. It certainly doesn't do a realistic job. If Wright is right, MI5 is something of a sexual Gobi Desert.

I suspect that the spy novel will survive, though it may never make the same claim to authenticity. After all, Hercule Poirot lasted even though his life was not exactly like that of a detective in Detroit. But it will need to become even more stylized than before.

Simon Hoggart is Washington correspondent for the London Observer, and the author of several books about British politics.