If power were just about size, Tesco's Sir Terry Leahy would top our Power List regardless, and the big four would follow in rank order.

But that would be a rank idea: not only dull, but wrong. Sir Terry's tops, of course. But is Andy Clarke, Asda's new CEO, powerful right now?

He has the mighty Walmart backing him. But that's hardly a cast-iron guarantee, as new Morrisons boss Dalton Philips could tell Clarke from his own experience with Walmart Germany in 2005.

With Asda's like-for-like sales down for the first time in four years, and the toughest trading conditions since the early 1990s, according to outgoing CEO Andy Bond, we place Clarke seventh in our Supermarket Power List. And there are equally controversial placements in our Power Lists in wholesale and convenience.

What is evident, from the landscape, is a power vacuum in grocery. This is an extraordinary statement to make in such a big, influential sector. But grocery is not only undergoing seismic change at the top, with new execs leading Asda, Morrisons, M&S and Aldi, and vacancies at leading foodservice giants 3663 and Brakes. It is also trying to steer a path at a time of enormous uncertainty, politically and economically.

That's why Tim Steiner, CEO of Ocado, has dropped a place in our ranking this year. With sales up 30%, and a potential IPO in the summer, Steiner's star should be rising in anticipation.

But how can it when the City is so jittery? The power is in the hands of the markets and, of course, the shareholders. And that makes JLP chairman Charlie Mayfield powerful right now. With many expected to hold on to their shares, a flotation would rely on Mayfield to sell JLP's 29% stake.

He's unlikely to say no. A £1bn valuation of Ocado would net JLP £290m on its £65m investment - not a bad return versus the recent cap-ex investments of the big four, don't you think?

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