In my part of the world, Oxford, Sainsburys have a major market share of both their own grocery & other in store products & fuel.
There are two large stores about 10 minutes apart, one north of the city & the other south. Over recent weeks the northen store has consitantly been 3p per litre (across the board) more expensive than the southern store, which has a Tesco 5 minutes away.
So if you're in this part of the world you might want to a) use the southern store, or b), exert some customer pressure on this inequal manner Sainsburys are treating their customers & buy at Tesco .... after all if fuel is so much more more from one store another, what else is at higher prices in store?
As a further guide to the level of injustice Asda, 10 minutes from either store is consistantly 4-5p per litre cheaper than Sainsburys cheapest price .... so who do the big boys think they're fooling?

I have always handed my Nectar Card but this store seems to always put less points per litre bought or forget to put points on and then advise you to put them on at the main store.