Ever since Samuel Pepys, that quintessential Londoner, published his famous diary in the 17th century, British readers have had a taste for diaries -- a taste now satisfied by their newspapers.

Most British publications today run some form of diary, ranging from erudite, arcane essays in the quality press to ordinary gossip columns in the popular dailies.

The august Times of London runs a daily diary written by several correspondents, and the lofty Financial Times carries one called "men and Matters."

The only papers without a diary are a couple of tabloids at the cheaper end of the journalistic spectrum. As one observer puts it," "They consist entirely of one long gossip column."

In recent years, Fleet Street's liveliest and most knowledgeable gossip columnist has been, by consensus, Nigel Dempster, 38-year-old bon vivant who edits the diary for the brightly written Daily Mail.

"Nigel is easily the best," one veteran Fleet Streeter said the other day.

"He's got the names, the facts, the style, the wit, and he creates a lot of controversy."

Yet Dempster, at the acme of his career as the Country's foremost commentator on marriages, divorces, and breakups, is planning to walk out on his job.

"I have decided to fire the Mail," Dempster said insouciantly while being interviewed at his desk in the paper's dingy offices just off Fleet Street.

Dempster, who has no small admiration for his own talents, added: "I have revived gossip journalism in Britain. But I've reached the top, and why should I continue repeating myself?

"In the United States, a gossip columnist only hits his stride after 40. Look at Earl Wilson in New York, Irv Kupcinet in Chicago and Herb Caen in San Francisco.

"But in England it is considered good form to retire at a reasonable age from the daily grind."

Dempster's decision to leave the Daily Mail is said to have been influenced by the fact that in addition to angering some of the many celebrities he writes about, he has piqued his publisher, Lord Rothermere. Dempster has sided with Lady Rothermere in their marital squabbles, which have broken into print in the last year.

Rothermere wrote a memo to Dempster which said in part: "Your column in the past few months has, in my view, deteriorated, and I commented to the editor that it has been showing all the signs of neglect.

"You are devoting far too much space to foreigners of no particular interest to our readership and a tired roster of well-known names.

"Your column, one might say, was beginning to have the taste of an old, cold, fried potato."

Furious, Dempster replied that he was congratulated regularly by opposition gossip columnists. Lord Rothermere's memo, he said was "quite monstrous."

Dempster added that Rothermere had asked editor David English of the Mail to fire him but that the editor declined because it would make the publisher "look silly" and cause "untold trouble" with his colleagues and the unions.

"The editor didn't want me to go," Dempster said. "Circulation will suffer. And if I could be tossed out, no one's job is safe. But I shall leave anyway when my contract is up on April 1."

Many Fleet Street journalists who frequent the Dickensian pubs and wine bars in the historic center of London believe that the Daily Mail has been foolish to antagonize Dempster, since a good gossip columnist can mean the difference between profit and loss in his competitive market.

Competition is particularly fierce among the national tabloids, the so-called "down market" press. But even the more elegant diarists of the quality Sunday papers, of the Daily Guardian, the Telegraph and the Times, have their loyal followings.

Dempster finds the diarists of the morning and Sunday quality papers either too esoteric or boring, or both. But he admits that Emma Soames of the Evening Standard, daughter of Rhodesian Governor Lord Soames, at least travels in the same league with him, as does Compton Miller of the Evening News.

Still, he insists that his column has the best pace and tone of any in Britain. He likes to include about eight separate items in each morning's column, along with several pictures.

"My idea is that when you have read one of my items you know who the people are, what their predigree is, their financial standing, who they have been sleeping with, and who they are currently going to bed with.

"And don't forget that readers like to know that the rich and famous have their problems, too.

"I also write about Gstaad, Paris and Hollywood," Dempster said.

"I call my contact in Los Angeles every day and Hollywood items appear in my column most days of the week. The place fascinates me."

While Dempster is regarded as an accurate gossip columnist, his column brings him his share of legal actions.

Dempster was born in 1941 in India, where his father was an Australian mining engineer of Scottish origin.

He was tossed out of a private school in England, and went to work at 17 for Lloyds of London. He soon shifted to an advertising firm, which included among its clients a vodka company.

"I was responsible for creating a whole new class of alcoholics," he says of his efforts.

He joined the Daily Express as a "legman," and by 1973 was chief gossip writer on the Daily Mail. Meanwhile, he contributed to the satirical biweekly Private Eye under the nom de gossip of "Grovel," and often referred in the Grovel column to the "notorious drunken fortune hunter Nigel Dempster."

Handsome, urbane and witty, Dempster made a point of cultivating the right people and attending all the debutante parties. A couple of years ago he married the daughter of the 11th (and last) Duke of Leeds. The marriage has given him an occasional problem, and he recalled an example:

"The Earl of Pembroke was giving a ball to which Prince Charles was invited. I was invited with my wife. Prince Charles was worried that I'd take a picture of him with some bird. But I gallantly said that I would be off duty that night."

Dempster puts in a full day, working out in the gymnasium of his club before going to the office between 10 and 11 in the morning. He then answers his mail and begins a daylong series of phone calls. In the late afternoon he writes the column with the help of an assistant or two, then lays out the page where the column will appear, and goes off for a drink, a party, a play or a concert.

"When I started," he reminisced, "gossip columnists were much reviled creatures. People were either at your feet or at your throat. Now I'm occasionally an honored guest at a party."