FREDERICK FORSYTH has become a rich man writing thrillers starting with The Day of the Jackal and proceeding on through The Odessa File, The Dogs Of War, etc. He has also become, apparently, a British Tory who regards the present Labour Party as a nesting place for Hard Left sleepers who will, on a given day, awaken to do the Soviet Union's bidding. He may have something there as anyone who has watched the farther-out activists of the current Labour party pursuing their joyless causes must acknowledge.

But Forsyth's new novel, The Fourth Protocol is not a hot-eyed tract. Frederick Forsyth is, first and foremost, an entertainer. His political proclivities provide him with basic assumptions on which his story is based; they are not obsessions which the story must serve and justify. As a reporter he has furnished his new novel with plausible-sounding descriptions of power's physical establishments -- British, Soviet and South African.

The Fourth Protocol (the title is derived from a subclause of the 1968 Nuclear Non-Proliferation Treaty under which the signatories agree not to smuggle nuclear devices into each other's territory) begins on New Year's Eve, 1986. Margaret Thatcher still governs Britain, she is preparing for one, last election coampaign against Neil Kinnock's wishy- washy Labour leadership, and there is a jewel heist in a fashionable London apartment house. The thief steals a briefcase in which to carry away his swag, discovers that classified documents are hidden in its lining and, patriotic British hood that he is, mails them back to Her Majesty's appropriate ministry.

This triggers an MI-5 investigation of who's leaking secret documents and to whom. Enter John Preston, midforties,a former Army Intelligence operator in Ulster, now employed as a second-echelon civil servant in MI-5. If The Fourth Protocal has heroes, Preston is one of them. He is an attractive, low-keyed man -- divorced, underpaid, harassed by the old networks and career ambitions of his masters.

The other hero -- in that he, like Preston, engages our interest as a man fighting a solitary battle against his country's bureaucracy and leadership -- is General Yevgeni Sergeivitch Karpov of the Soviet KGB. Karpov, a specialist in British affairs, discovers that an important anti-British caper is in the Moscow works and that the KGB has been excluded from it. Boring in from the outside, Karpov finds out about Plan Aurora -- a scheme concocted by the aging (and unnamed) General Secretary of the Soviet Cummunist Party and four confidantes -- including a by-now decrepit Kim Philby, the British traitor who defected to the Soviet Union in 1963.

Under Plan Aurora a Soviet agent is smuggled into Britain. He is supposed to detonate a small nuclear device near an American air base a few days before the forthcoming British General Election. In the least plausible part of Forsyth's plot, the bomb blast is supposed to scare 10 percent of the British electorate into voting Labour. Once in power, Neil Kinnock will be deposed and Britain's first Marxist- Leninist prime minister will take over, unilaterally disarming the country, kicking out the Americans, withdrawing the U.K. from NATO and otherwise making the old goats in Moscow drool with happiness.

Back in Britain, John Preston is supposed to uncover the the plot before it's too late. As usual, no spoiling the fun here by tellng you how it all comes out.

As a plotter, Frederick Forsyth tends to rely a bit too much on unexpected or coincidental events -- Scottish hooligans accidentally stumbling over a Soviet courier late at night and beating him up, the jewel thief swiping just the right briefcase, etc.

But this and the questionable premise about mini-nukes changing the temper of the British electorate are compensated by something new in Frederick Forsyth fiction; The Fourth Protocol has people in it unlike the one-dimensional characters of The Day Of The Jackal and The Dogs Of War. They are interesting people, even the repellent ones. Four books and a few million pounds after Jackal Frederick Forsyth has become a well-rounded novelist. The Fourth Protocol is his best book so far.