Just to update this thread - I saw 36mpg on the way to work this morning - ok I drive like a granny most of the time, but even so!
About 250 miles done and it is almost halfway between 1/4 and 1/2... Im hoping it will stretch to 350 miles, that said I never leave it low, it's my first tank in this car so will probably nip to Halfords at lunchtime and buy some sort of fuel cleaner / additive.