supermarket petrol - can anyone answer this?

It might be that they recently changed that tank to a different fuel.

Yes, we ask the customer! Quite simple really.

No mystery really, just ask me. If you choose not to regard any of my data, that's fine too.

Since the pumps are all marked Esso, I'd think they'd get rather annoyed if some rubbish was sold from them?

All the petrol in a particular area comes from the nearest refinery. This means that in Edinburgh where I live the petrol comes from the BP Grangemouth refinery, even the Esso, Mobil etc. I think that's how it works.

Esso around here comes from Conoco (Jet).

No, all the unions are the same. Suppose that it could be possible that the wrong stuff was put in the wrong pot at the refinery assuming the driver didn't c*ck up.

On a quick note here, i can categorically tell you that the fuel is exactly the same as delivered to different companies stations. All the "difference" might be is when a driver gets up on the tanker and pops a vial of magic essence into the tanker chamber if it hasn't been done pre-filling. Scottish oils for instance is a BP company but they allow Texaco, Q8, Shell and a few other companies to fill up. I'll leave the rest to your own conclusion.