FWIW had a chat with a tanker driver. His take on Tesco was that their petrol (can't vouch for diesel) is mainly imported, Russia was mentioned, and quality is such that it just meets the requirements. Other supermarkets, Asda, Morrision's, source their supply from refineries direct, Conaco mentioned, so quality is higher.
Of course all this may just be bull but there must be a reason why 'branded' fuel is always recommended (and as said if near supermarket generally price marched to the same cost).
Just a few thoughts
