No Power

[GUnit]

Hi, I am new to the jaguar X Type. We acquired the car 2 months ago  and have since had it serviced it has done 78200 miles and it is a diesel   It seems to have very little power in the mid range ?is this correct? It has had a problem with the injectors before, it was used very little for a time probably only doing 3/4 miles a day. since we have had it, it has been used to do 50 mile journeys motorway and b roads. On a journey recently of 50 miles it only returned 34miles per gallon? again could this be right.

Thanks for your help in advance.