So winter is here and petrol price just keeps going up

I wanted to ask if you guys feel I had a problem with my 2003 Cooper Auto as I just calculated that for the past 3 tanks, I am only getting 26.7mpg (UK). I do 80% town and 20% motorway driving. I looked up Parkers and I should be getting 31mpg extra urban. What do you guys think and what mpg's are you getting?