Looks like Tesco's have upped the ante already as they are currently offering whopping 10p off per litre of fuel if you spend over £60 (corrected from £40 in the original post, thanks for the feedback Anonymous!). Thats around £5 of a full tank of petrol! And to cap it all they are also offering a further £5 off your next spend over £40 in store.
A top tip with these deals is to go to the store and only buy the minimum amount you need to qualify for the vouchers. If you don't particularly need any groceries then you can always stock up on a few non perishables that you'll need to some point. Things like a family pack of washing powder, some toilet paper, perhaps things like toothpaste and shower gel. Before you know it you'll have spent enough to qualify, and then you can keep the goods you just purchases until you need them.