[ThomasG]

Hmmm... Fully logical.
But.

After return, I started "experimenting" with different brands of fuel. My average mpg was climbing, two of best being Sainsbury's super and shell 99octane. Mpg went high, as for this car. Record shown by on board computer was 26.6 mpg.
Then.. I filled premium at station with green flower.
Almost instant drop to 23.6, and after using up that tank, I struggle to bring it over 25 no matter what I fill.
So.. apart from tarmac..
Is it at all possible that European petrol has additives that help engine, with effects lasting for about 3 weeks after return to UK.

Or is it more possible that something went wrong with my engine same time I filled that premium, not necessarily caused by fuel at all.. and now it burns more..