Ask them to check the price. The man who I got the heads up from saw the deal at a Tesco fuel station in the Midlands so went to his local Tesco petrol station back home and the same deal was on there too. I went to two stations near me and bought a total of 8 litres. One of the garages showed it at full price but when I asked them to check it, it came up at £3.12. Worth the ask as I can't see why it should some garages and not others.