SIR MARTIN SORRELL is on the phone. To me. He is too busy to meet — firms to run, figures to check, clients to squeeze — so with the interview blown out, he has slotted my call into a hectic schedule. First 2pm is promised, then 4.30, finally 6. And being many miles apart is OK because we've met before, and while seasons change and business ebbs and flows, Sorrell, chief executive of WPP and one of the longest-surviving bosses on the British stock market, is immutable. Short, courteous, brainy, always in plain shirt, silk tie and lightly striped double-breasted suit, he is, when we finally talk, sitting in the same first-floor office in the same understated mews house in Mayfair from which he has run his marketing empire for a decade-and-a-half.

Maybe, as it's after 6pm, he has taken his jacket off, a small concession to a long day. In manner, however, he is convivial. He asks if I mind him eating chocolate — "got to get my energy levels up" — and punctuates our talk with the sounds of frantic masticating, topped by the occasional lip-smack.

Sorrell, 58, reserves the capacity to surprise. Modest in appearance, ambitious in his aims, he remains a business conundrum. Happiest out of the spotlight, he spins away quietly, a busy spider sitting at the heart of an ever-larger web, until the next purchase flies into view and his extraordinary avidity is exposed again.

The latest target is Cordiant, the struggling services group that emerged from the ruins of Saatchi & Saatchi, minus its founders, in 1997. Wracked by debt and a plunging share price, the group appears to be wrapping itself up nicely for the WPP boss. He wants all of it, he says, if the price is right. Why?

Munch munch. Because the Asian businesses, the healthcare marketing and Fitch Design are a nice fit, he says. But particularly because of Cordiant's expertise at in-store marketing. Sorrell spends a lot of time listening to WPP clients, teasing out their worries, and he has spotted a trend. "Many companies believe that up to 80% of brand decisions are now made in-store."

So WPP needs to beef up that side. And that's what Sorrell, boss of a marketing-services empire with 62,000 employees in 102 countries, does when he is not studying the daily cash balances. Talks to people, analyses trends, works out what's coming next, and buys to fit. He wants less reliance on above-the-line advertising, a third of his business from Asia, everything in order for the growth in world economy that he predicts for next year. It's all rationalised like one of those studies from Harvard Business School where Sorrell is an alumni.

But gut feel must play a part. If he gets Cordiant, he's closer to overtaking Omnicom, the biggest ad group in the world, and he can hang his latest kill next to his other trophies: J Walter Thompson (bought in 1987), Ogilvy & Mather (1988), Young & Rubicam (2000). Surely it's only a matter of time before he gobbles up the Saatchi brothers themselves? He used to work for them as finance director, in the days when Maurice's glasses were even bigger and Charles was not yet Garbo and Guggenheim combined. Wouldn't running them be a neat twist? Perish the thought.

"That," says Sorrell gravely, "would be more trouble than I could deal with." He laughs. The brothers were not, as many think, the main influences on his career, he says. "No, I think Mark McCormack was, and my father." Sorrell's father ran a retail group; McCormack, the silver-tongued sports promoter who died this month, employed Sorrell early on in his career and showed him how to cut deals.

McCormack once described Sorrell as "bright, but not what you would call a glad-handing, people-person", and some since have found him hard to work with — one boss memorably described doing a deal with Sorrell as "like trying to stab a dolphin with a banana". Yet he has gone out of his way to cultivate other company chiefs, journalists and academics. Because? He likes to be kept informed.
"My approach is a little intuition and lots of talking to people. Our business is like investment banking or consultancy — you get to meet a huge number of clients with different problems and varying approaches."

Others, noting his old-fashioned suits and reticent demeanour, have speculated that he might have found banking a better fit, but that underestimates his smartest decision: to push into a sector where few share his skill set: "I never believed the ad industry was about art or craft. This is a business." Beneath the unflashy exterior, he has always oozed confidence — an only child born to doting, wealthy parents, it was engendered young, along with a determination to succeed that he ascribes to surviving a frightening car crash in his youth. You can still see the scars on his broken nose.

Since then he has brazened out share-price slumps, two restructurings of WPP's billion-dollar debt, and complaints about his salary and benefits, with barely a shrug. A £45m "golden parachute"? Rubbish, says Sorrell, suddenly terse. "The 'parachute' I have is one approved by shareholders in a specific vote." He estimates it at about £2m. The rest is WPP stock accumulated with his own money — 13.5m shares. And he has never yet sold one, he says proudly.

One day, he'll have to, but asking when is like asking if he'll ever step down. "I suffer from Founder's Disease," he says, relaxing again. "Maybe someone will tap me on the shoulder and say 'time to go', but . . ." Don't bet on it. He has survived so well that one banker recently described him as having "balls of titanium". They'll be clanking again today when he opens the batting for WPP's cricket team in its annual match against O&M should someone dare bowl him out. Cricket is a hobby, but played with characteristic intent.

Who chooses the team? "Me." First in to bat? "Me, of course." Sorrell laughs again. In the end, you can't help feeling that the greedy goblin of adland legend might surprise McCormack with his people skills these days. "I am so sorry to bugger you about earlier," he says, concluding our call. "God bless."