Supermarket fuel does seem to be worse I agree.
I personally believe the problems all relate to the new low sulphur diesel fuels and there associated bio fuel content. The black stuff you see in filters is known in the marine trade as Diesel Bug, a bacterial growth which lives and grows in the interface between diesel and water; all diesel contains a small amount of water, we are talking parts per million here. Now I am guessing cheaper fuels contain more water, and following lots of research by marine trade bodies we know that add bio fuel to the mix and bacteria growth goes into overdrive!
The marine trade after much legal fighting with the goverments and the EU can still supply old style high sulphur no bio fuel red diesel after the safety implications of diesel bug were highlighted.
I know from a mate who works in a large HGV servicing outfit that even lorries now have issues with diesel bug so at some point the problem will occur in cars and landrovers irrespective of mileage. Location may be an issue as well, quiter stations with stagnent fuel in the tanks or old tanks may well have the bacteria growing in the storage tanks and be contaminating fuel which arrives clean.
Personally I add fuel a marine fuel treatment to the diesel which goes into our car and the landie, after using 3 tanks in the zafira the MPG climbed by 7% using the same fuel station so it does make a difference