Cast your mind back to February 1997. Tesco had just taken top spot in the grocery rankings, but at about 20%, its market share was only 2 percentage points above Sainsbury’s.

Cast your mind back to February 1997. Tesco had just taken top spot in the grocery rankings, but at about 20%, its market share was only 2 percentage points above Sainsbury’s. Tesco had just made its first tentative steps into Eastern Europe, was poised to open its first Extra store, in Essex, and a month later it was to make its debut in Ireland by acquiring Quinnsworth.

All were landmark events for the retailer, but put in the shade by one other that month - the start of the Terry Leahy reign as chief executive. He inherited a tremendous legacy from his predecessor Ian MacLaurin - one Leahy himself had contributed to, not least with the launch of Clubcard two years previously - but has built on it to indisputably become one of the all-time great retailers.

The secret of Tesco’s success under Leahy has been simple. It is the unrelenting focus on the customer - how they think, how they behave, how they differ from one market to another, from one town to the next. To simplify the recipe might appear to be doing Tesco a disservice, but developing the depth of understanding that Tesco has of its customers has been key to its ability to move into non-food, international, services and online so effectively during Leahy’s time in charge.

Leahy has created a culture that keeps Tesco both modest and hungry, exemplified both by its dowdy Cheshunt headquarters and by Leahy himself. It may be number one by miles, but it still thinks like a number two.

Like any successful leader, he’s had his share of luck. For large parts of his reign he benefited from his biggest competitors, notably Sainsbury’s, being on the back foot. And not everything has been plain sailing - the jury notably remains out on US venture Fresh & Easy. But overall, even his rivals would acknowledge Leahy’s achievements, and perhaps quietly be celebrating his departure.

As when Leahy replaced MacLaurin, the appointment of Phil Clarke as Leahy’s successor has been seamless. Spending most of his time travelling around as head of Tesco’s international business meant he has been on the radar less than other succession candidates, but Clarke is very much from the Leahy mould - he’s been at Tesco even longer, since 1974 - and like the man he is succeeding, he is steeped in the Tesco way.

That doesn’t mean he will find it easy, and with so many other Tesco lifers on the board, others may be disappointed at missing out. The upside of this is that while Leahy will be missed, it should be business as usual, and Clarke should prosper - after all, he’s had a good training.