Sadly, Tesco reported a mixed set of figures today << BBC report >> with Sainsbury's a little better.
The BLOKE they interviewed (sorry, I was experiencing a Marmite problem just at the same time) did mention that Tesco Research Institute for Customer Kontrolle (T.R.I.C.K.) pointed out that whilst main store sales had sufferred, those babes in the Tesco Express stores had been going nineteen to the dozen, the poor bunnies.
It seems shoppers are buying less at their 'main' shop - and topping-up more.
He said this was a sign of prudent shopping, and cutting down etc etc - I suggest it's a sign that people haven't learned to shop 'properly' at all in the last 20 years, try to 'cut down' (haha . . . read << this >> if you've time ) and then have to pay MUCH MORE as the student-sales fuelled Tesco Expresses mop-up the same targets from another gun emplacement.