For the "supermarket saving", did you include Asda in the supermarket pool, or as a general pool? They seem to be rather less price competitive than other supermarkets, I'd presumably because of the recentish private equity takeover involving petrol station operator Euro Garages meaning they've kinda opted out of the petrol price war (they're hardly likely to want to undercut their existing forecourts).

Although the other recent private equity takeover of Morrisons led to some sort of deal with Motor Fuels Group to operate their petrol stations (but no ownership stuff in this case?), but they're seemingly still being competitive with Sainsbury's and Tesco's.