Spotted this at Tesco's in Crawley, not sure if it's nationwide. Seems to be a bit of a reverse of their usual '5p of per litre of petrol when you spend £50 instore'. Deal is:
Spend £20 on petrol get 50p off shopping
Spend £40 on petrol get £1.00 off shopping
Spend £60 on petrol get £1.50 off shopping
I didn't see any mention of a minimum spend requirement.
Admittedly, not huge savings, but as Tesco says 'EVERY LITTLE HELPS!!'