I would check your coolant temps first before having unnecessary work carried out.
Sounds low MPG to me, I normally get about 575-600 miles to a full tank, £80, although mine has been re-mapped.
I would say driving style and roads effect mpg more. My OBC mpg sits at 42 but I normally have the range on the displace with the trip zero'd when I fill up. So as long as the 2 sets of numbers adds up to over 600 then I don't worry too much about the economy as this is about 45mpg.