Supermarkets frequently use fuel as a loss leader to get people into the store. They make a lot more profit on £50 of groceries than on £50 of fuel.

Shell, Esso, BP promote the idea that their brands are specially formulated and worth paying more for. They may sell sandwiches and coffee, but need to be profitable on fuel. So they charge more than the supermarkets.

Most of the supermarkets price locally. Sainsbury's at Fosse Park in Leicester is always 2-3p a ltre cheaper than Northampton.

I suspect that the difference is the presence of Asda who price nationally.