Clive Sinclair, a slender, bespectacled 42-year-old Englishman with a shy smile and a newly fat bank account, sees big changes coming in the world, and offers a stunning forecast that, if true, will affect everything we do. "The 1990s will differ from the 1970s as profoundly as the 19th century from the 18th," he told a British television audience recently.

With the advent of the silicon chip and the robot, he said, society is on the edge of economic transformations comparable to those made by the automobile, telephone and airplane. The new technologies, he predicted, will create "truly intelligent machines" that serve our every need, even medicine and law. And traditional manufacturing jobs will largely disappear in advanced countries like Britain, as China, India and elsewhere become the great producing nations, because labor there is so cheap.

By the 1990s, Sinclair said, "We must turn from the products of the material to products of the mind; books, video tapes, television programs, computer programs, design services... where others produce we can design." If all that seems implausible, Sinclair argued, remember that 60 percent of Britain's labor force were once farmers. Today, less than 2 percent work the land and provide far more food.

Sinclair's authority on such matters is well-earned. Since leaving school at age 17, he has struck it rich -- and occasionally gone bust -- by being first in the world with dramatically advanced ideas in technology, most recently the cheap personal computer, selling for as little as $80. In three years he overwhelmed the market by selling hundreds of thousands of computers around the world the way Timex Corp. once dominated the sale of low cost watches -- which is not accidental, since Timex now has the license to produce and sell Sinclair's computers in the United States.

In the last five months of 1982, Timex sold 600,000 Timex-Sinclair 1000s in the United States. Largely because of Sinclair marketing, Britain today boasts twice as many personal computers per household as the United States, and many times more than France or West Germany. "You've got to be pretty excited by change," he said in an interview, debunking the myth of diehard British resistence to new things, "to go through all the hassle of learning to use these damn things."

A decade ago, it was Sinclair who developed the first pocket-sized electronic calculator. He was a pioneer in the design of digital watches and produced the first miniature flat-screened television, an updated version of which, he says, will go on sale later this year at less than a third the price of the super-popular Sony Watchman.

Sinclar's cool intensity and his willingness to take risks show him to be as much an entrepreneurial packager as he is an electronic innovator. He is also very much a man of this world, with an interest, he says, in poetry, music and high-speed cars. In London, Sinclair lives alone in an impersonal furnished apartment, his living room distinctly cluttered with technical gadgetry.

Recently Sinclair pocketed a check worth nearly $20 million, made out to him by N. M. Rothschild, the celebrated London bankers. The fortune was all the more impressive because it represented only 10 percent of the value of Sinclair's privately held computer company scooped up by investors when he decided to sell some shares to raise cash for yet another pet project -- an electric car.

In a way, Sinclair Research Ltd. is a prototype for Sinclair's vision of the future for Britain. With only about 50 employes, mainly working out of a warren of offices across from King's College in Cambridge, the firm is having a profound impact on computer technology, enough to attract big name competitors like Texas Instruments and Commodore into making down-market models intended for home use.

On his record, Sinclair is best at fast starts. The Japanese eventually overtook his calculators by making them better and cheaper. He lost valuable time in the digital watch race when there were problems with some of his components. And an early version of the pocket television floundered because of weak marketing. Britain's National Enterprise Board eventually sunk $17.3 million into Sinclair Radionics, which folded when Sinclair pulled out.

All that explains why Sinclair, who reconstituted himself in 1979 as Sinclair Research, now concentrates on thinking things up and basically letting others do the making and selling. It also gives him more time to think about how to service the way of life he is so certain lies ahead.

In his provicative brodcast for a BBC "Futures" series and in a conversation at the London apartment where he stays when not in Cambridge, Sinclair talked about the painful transition that Britain, in particular, and also the United States must go through before reaching his imagined millenium. Unemployment already exceeds 3 million here -- about 14 percent -- and has the potential of rising to 7 million in the next decade if opportunities in manufacturing drop as low as Sinclair expects.

"Daunting this may be, disastrous not," he said.

The great industries of the West that produced the economic explosion of the 20th century -- automobiles, airplanes, plastics -- "every one of them is at least 50 years old and they have reached maturity," he said. For instance, "the United States is the comfortable world leader in making planes, but nobody wants any more planes... the new jets that Boeing are offering are only very, very marginally better than the jets they have already been selling. There is no profound incentive to buy."

Moreover, the Japanese, with a new auto industry, are able to achieve in efficiency what the United States and Britain with their "mature" ones no longer can, he said.

"The truth is," Sinclair told viewers, "that Britain is the wrong part of the world to make conventional goods. I believe that products with a strong creative content can be made here profitably. As soon as a product becomes standardized, by evolution, as most cars have, it is best made in less sophisticated, and so cheaper, countries to our mutual benefit -- the benefit will be ours since the product will be cheaper. And they will benefit with increased opportunities for employment, profit and export..."

Britain should go on making specialty items like the Rolls Royce and Lotus cars as long as demand continues, but the new jobs, he said, must be in innovation, new aras that a computer-fostered economic regneration will create. Britain is already the world's largest exporter of "services" like banking and insurance and that should go on developing, he said, as well as an expected boom in the creative enterprises -- television programs, for instance -- where Britain is an acknowledged international leader.

But technology, as Sinclair sees it, is the great frontier for jobs that will emerge as computers lead to intelligent machines, biotechnology, solar energy, thermal power, leisure parks, health care and so on: "Once you can get a robot that can do anything in practical terms that a man can do, but do it for less money, then you have made a change of an extraordinary nature. Because one robot can make another and it's got a multiplying effect. And then the sort of things dismissed in the past as being totally uneconomical suddenly become totally possible... like putting up vast cities in space... it will be straightforward."

That there will be work and wealth, Sinclair has no doubt, but he acknowledges that guessing where it will all come from is impossible. "We are a particularly creative people and live in an exceptionally free society," he says of Britain; "government must take a truly laissez-faire approach to the new businesses," and not seek to take responsibility, as has happened here to the declining older industries like steel or shipbuilding.

Plainly, upgrading education for future needs is critical, and Sinclair believes that will happen naturally, again with the benefit of computer-led breakthroughs.

"There is a tendency for people and politicians to resist" the sort of change Sinclair regards as inevitable, "and to imagine that they can bring about the future by planning it. But really, it can only happen with the free operation of the market place." The risk, therfore, is that the new "Golden Age" Sinclair forsees, a time of immense benefits for all, will be delayed -- or worse -- because it seems too radical. But the alternative, he says, is inexorable decline. The future, like the tide, is unstoppable.