Got back from Normandy just over a week ago. Diesel prices varied quite a lot with prices ranging from 0.96 to 1.38 Euros. As with the UK, supermarkets seem to be the cheapest.
There are signs for Supermarkets like Carrefour, E'Leclerc, Super U and Intermarche all over the place but they take a bit of understanding and they don't always show if there is fuel. They give the town where the supermarket is and then the road that the supermarket is on. But they don't give the road name they give the route to another town. You therefore follow signs to the main town where the supermarket is and then follow the signs for the second town. The supermarket will then be on the route to the second town.
It sounds complicated, but it works remarkably well. I found the Decathlon in Le Havre this way and then spent a load of money on cycling jerseys.