307sw 1.6 hdi110, miles per gallon?

Hi,
I have the above car. Everywhere on the internet it says that it will do 
around 50MPG (UK)There are lots of reviews where people seem to be getting

that sort of mileage. What a suprise - mine doesn't get above 42 and if I 
drive normally I get 38. It has done 80,000 miles. It has been serviced 
properly from day one and even when driving on the motorway I am getting
42. 
Is this normal? Could something be wrong and what can I do to get more 
mileage (Without driving like an old man)? what difference could the fuel 
filter make?
Cheers