>>> Surely they're not all supplied by the same tankers?
Back in my trade plating days (delivering cars across the country), I hitched a lift from a tanker driver. At that time there was a lot of debate as to whether supermarket fuels were as good as those from other filling stations. He said it had to be, as he delivered exactly the same fuels to all of the big supermarkets, plus, Shell, BP, Esso, Jet (etc) garages.
There are actually only a very small number of fuel depots in the country, with each of them supplying nearly all of the filling stations within a fairly large region. So, when one filling station runs dry for some reason, it's not unusual for other filling stations in the area to have similar problems, even though their fuels are all branded differently.