Being in the U.S., I don't know for sure what Tescos is, although I've seen it referenced here and I assume it's a chain of stores. If that's the case, it's highly unlikely that they also refine their own products. It could well be that British Petroleum, being the giant that it is, could be the supplier of the diesel sold at Tescos. Fact is, there are relatively few refineries worldwide and smaller distributors of all petroleum products buy them on the world market so there's no way of assuring who is doing the refining and blending...