My friends dad is the Director of Science for Shell Uk, I had a discussion with him (a drunk one) regarding fuels and the basics are that 90% of fuel comes from Shell & BP, supermarkets such as Tesco etc buy the fuel directly from the major suppliers and sell it on. Supposedly BP make more money selling fuel to Supermarkets than they do in there own forecourts.
I had a disagreement with someone a few months ago who was slating tesco fuel, how rubbish it was and how tesco should do something about it at there refinery, I had a laugh saying 'Do you honestly think Tesco has an oil rig in the North Sea pupming out oil, possibly with a metro store too incase you want munchies while driving past!', I got a funny look.
So to clarify, all the fuels are basically the same, only the high performance fuels such as Optimax and Tesco 99 (engineered by another supplier) have different characteristics. I think we just hear more about bad supermarket fuels as the biggest majority of people use them to fill up while getting the weekly shop, but its not unusual to get a bad batch from BP / Shell either.