Depends how they store the fuel and clean tanks, from memory the was only Tesco that had a major issue a couple of years ago.
Even the tankers transporting the fuel can be contaminated buy water or cross fuel, diesel in petrol or visa versa and if super markets use there own trucks or dedicated contractors that is how they have fallen for the contamination, it would be more widespread had it originated at the refinery.
Think, if each chain had its own refinery we wouldnt be able to move for them all over the place
