Citroen C5 2.2 HDI Estate

Hi, I have bought the above car in November, Its done 60,000 miles.I love the car in every way except for the fuel economy. I am getting. My computer trip meter packed in 2 weeks after I got the car, & eventually managed to claim off my warrenty, to the tune of approx £600 ish. They fitted a complete stalk unit onto my stearing column. This got me up & running again with all the gadgets, fuel consumption etc.Is there any way this job could have affected the engine management system, because I hadnt had the car too long before this job, I cant compare the consumption before it went in. If I run the car at 67mph in cruise controle on the motor way, I get approx 45mpg. This sounds reasonable, but if I check my consumtion on all motoring, ie motorway & town driving, I get approx 35/36 mpg. I dont know if I am being too faddy. I dont want to waste my money on a £70 engine diagnostic test. I love the car in every other way. Any comments please

Hi, the motorway/town driving, is approx 35mph or slightly less. I must say I do not drive the car very hard iether. In fact I drive steady. What do you think??
Thanks for the reply by the way