You must have a dodgy Esso garage Dave
my 940 loves Tesco and Esso 99 as they're ethanol free in the West Midlands but most of the UK still gets ethanol according to their website.
I used Tesco last fill up and I've done nearly 400 miles with a quarter of a tank left, I did a run down to Goodwood and back, definitely wasn't trying to get economy with my driving style and have done about 50 miles of town driving too so when I fill it up I'll see what the mpg was.
It will improve again once I track down the exhaust leak before the lambda sensor I imagine.