Sir Martin Sorrell of London is an empire builder much in the manner of another Briton, Baron Robert Clive of Plassey.

It took Clive two decades to consolidate a handful of squabbling Indian states and create the British Raj, which was to flourish for nearly 200 years.

It’s taken Sir Martin much less time to start the WPP Group in a one-room rental in London and expand it to more than 2,000 offices in 106 countries — possibly the biggest umbrella group of advertising and marketing companies in the world. Clive had barely 2,000 men at his command; Sir Martin has 92,000 men and women. In today’s dollars, Clive generated perhaps $50 million annually in land revenue and tribute from 18th century Indian maharajahs; the WPP Group’s worldwide revenues in 2005 were nearly $10 billion, some 40% in America, and its market capitalization is nearly $15 billion.

As he marks the 21st anniversary of an empire on which the sun never sets, this is what Sir Martin has to say: “There’s more to come.”

Clive would have approved of the statement. It is, after all, indicative of the brio that unreconstructed adventurers possess. And Sir Martin is nothing if not an adventurer. He’s a shrewd man who parlayed an education at Cambridge University and the Harvard Business School, as well as a keen sense of spotting undervalued companies, into a career that, at 61, has him pretty much perched atop a ferociously competitive industry.

“More,” in Sir Martin’s projections, means further expansion in Asia, particularly China and India — countries he calls “hot areas.”

“More also means more Latin American, more African, more Middle Eastern, more Eastern Europe, more measurable, more direct, more interactive, more Internet, more research-driven ventures,” Sir Martin said.

“More,” of course, also suggests more acquisitions.

“We are a multi-brand company that grows organically — and we grow through acquisitions,” Sir Martin said.

Those acquisitions include legendary names such as J.Walter Thompson, Young & Rubicam, the Grey Global Group, Hill & Knowlton, and Ogilvy & Mather. Not bad for a man who started entrepreneurial life by borrowing $1 million in 1985 and investing in a supermarket basket manufacturer called Wire & Plastic Products — the forebear of today’s WPP Group.

As Sir Martin tells it, the idea of entrepreneurship was embedded in his sensibility as the only child of a retailer, Jack Sorrell, and his wife, Sally. The elder Sorrell gave his son a job in the credit department of his company when he was 16.

The father’s boss, Sir Charles Hayward, took the young boy aside one day and said: “Martin, my boy, what are you going to do when you grow up?”

The answer: “I’m going to be a businessman.”

“Then you must go to Harvard Business School,” Hayward said.

That bit of advice stuck in the boy’s mind as he coursed through Haberdasher’s School, and then Christ’s College at Cambridge University, where he majored in economics.

Was he a diligent student?

With typical British understatement, Sir Martin said: “I always worked hard — but to no avail.”

Nevertheless, his grades at Cambridge proved good enough to get him admitted to Harvard Business School.

“I was ambitious,” Sir Martin said.”I liked America immediately.”

That liking has deepened over the years. So has his ambition, which has predictably drawn fire from business rivals. Their tart remarks have sometimes seemed to go beyond the pale — as when one characterized Sir Martin’s tendency to micromanage as jejune.

Did such criticism bother him?

“It doesn’t affect me,”Sir Martin said. “After all, we took on the establishment and prevailed, didn’t we? Maybe if I’d come from within the advertising and marketing industry, there would have been better acceptance of me. But it doesn’t matter. I live by the simple credo ‘Get it done!’ And yes, I do micromanage — I see that as a positive thing. In any event, all triumphs are transient.”

Sir Martin assesses business triumphs, and his stewardship of WPP, in highly personal terms. “WPP is my baby,” he said.

Among his early employers after Harvard were two men who also legitimately regarded their company as progeny: Maurice and Charles Saatchi. They had founded their eponymous ad agency in London and were two years into business when Sir Martin became their first finance director. He’d already had experience with an American consultancy, Glendenning Associates, Mark McCormack’s International Management Group, and James Gulliver Associates, a company that dealt in management advice.

“In all these jobs, I profited from the focus, intensity, and determination that I’d gained during my time at Harvard,” Sir Martin said.”Harvard’s hothouse atmosphere stayed with me. Fear of failure drove me. But the trouble was we were made to feel that we could run the world.”

He found out in due course that in order to run the world, one had to first run one’s company well. “To move from starting a company, as I did with the WPP Group, to running an international organization — that meant getting the balance right,” Sir Martin said.

That also meant wondering if he was investing in the right companies during his acquisitions spree.The 100 or so entities that he brought under the WPP umbrella have mostly fulfilled their financial promise.

It meant cultivating top management of acquired companies in a careful manner.”There’s no point existing if people don’t work together,” Sir Martin said. “But you’ve got to know which battles to fight and which to avoid.”

One battle that he dealt with deftly concerned David Ogilvy, who had said uncharitable things about Sir Martin when informed that the latter had launched a hostile raid on his venerable ad agency. Sir Martin invited the grand old man to be WPP’s chairman.

Ogilvy wrote the following note to his nemesis: “When I heard that you were trying to take over my company, I called you ‘an odious jerk,’ or words to that effect. At that point, we had not met. Then we met. … To my surprise, I liked you. Two English adventurers at large on Madison Avenue. … It remains for me to tell you that I am sorry I was so offensive to you — before we met.”

Ogilvy accepted Sir Martin’s offer.

What did the episode teach him?

“Battles of opinion are often only short term,” Sir Martin said. “And it’s usually wise to just move on.”

Clive of Plassey, who had his share of confrontations with jealous British parliamentary foes after his colonial successes in India, would have surely agreed.