I'll agree with Singh there on the BP.
I have some diesels at work and we run the big 4 fuel cards (BP, Shell, Caltex and Mobil). I find we always get better fuel consumption on BP. Three of the vehicles are the same make/model, all do basically the same in driving conditions and with different drivers refuelling at different servos you notice at the end of the month which gets the best result.
I personally use BP, but that is because it is around the corner and their prices are fair.