Tom, there are so many variables to result in poor mpg.
First is the weight of your right foot, are you one of the "heavy" lot, who hit the floor to get quick acceleration, or are you "light" footed and gently build up speed, and as importantly, how do you slow down, by hitting the brakes only when you have to, or reading the road and slowing well before the actual stop position.
Secondly, do youo drive with windows open a lot of the time, or aircon ON a lot of the time, both increase fuel consumption, quite dramatically. Thirdly, do you have a roofbox, or roof bars fitted? again, they reduce fuel economy.
Lastly, as previously atated, fuel figures are given as a guide ONLY, and arrived at under strict test track conditions, NOT round the normal roads, and no fuel figures include drving at 60 to 70 mph for prolonged periods, it is well known that the optimum speed for best fuel consumption is 55 mph.