Little details really do count in retail. And they can turn around the performance of a £40bn sales supertanker in a matter of weeks, as a close examination of Tesco’s performance in the latest four-week Kantar data reveals.

While the ‘headline’ 12-week performance was again flagged up by the national media to point out Tesco’s sub-30% share (the worst in seven years, it repeated), the four-week picture suggests that the measures taken to address its shocking Christmas showing have already had a quite dramatic effect.

Whether it’s till roll (up 1.1 basis points), grocery (up 2.0 points), or food and drink (up 1.7 basis points), the direction of travel is clear. And it spells bad news, in particular, for Asda (down 1.7, 2.2 and 2.2 basis points respectively).

So how has Tesco turned it round? In last month’s GPI, we showed how it had kept a lid on base prices (thus addressing concerns about its overall price competitiveness), while the net price (ie including promotions) had been allowed to rise through decreased promotional intensity.

But the turnaround in its performance is all the more remarkable when you consider that, as this month’s GPI reveals, it has spent the past few weeks preparing for its 29 February year-end, and the latest phase in its Big Price Drop, by, er, putting up its prices.

But the key to Tesco’s improvement, I believe, has not just been micro-managing millions of prices. As I said, it’s about little, simple details. CEO Philip Clarke identified that Tesco had not responded enough to the generous £10-off voucher schemes launched by its rivals in the run up to Christmas. So he instantly announced a £10 voucher. And combining it with 10 per litre off fuel, for three weeks in February, has delivered a swift one-two - so much so that Morrisons has abandoned its free shop promotions, and is offering 15p per litre off fuel.

Amid the noise of constant promotional activity - which underscores the success of Britain’s Biggest Brands - these extra savings must have come at a price. And they haven’t helped Tesco in terms of this week’s Grocer 33. But the Tesco tanker has turned. And in its wake there is turmoil.