They (all, not just supermarkets) seem slow to lower prices. I know they claim that they bought their stock at a higher price, blah blah, but they are always quick to raise prices inline with everyone else.
I think fuel is pretty competitive, and the actual retailer (supermarket or otherwise) makes about 10ppl, so an average of about £4 per punter, based on the 40l average fill (40l? I believe in only going to pumps when I really, really, REALLY need to! Hence, rarely less than 70l).
Clearly, the oil companies are still creaming it in currently, so they will get zero sympathy from me when oil prices are decimated. And the government creaming off the majority, well, I suppose they have to pay for all the lame, lazy, leftie do-gooders not to work