Welcome to the forum Matthew - good choice of car by the way
I would have thought you should get better mpg than that. I have a 2.0
petrol Grand Espace, the computer generally shows ~19mpg because most of the time itis used by the missus on the school run, all around town etc. It can get up to 30 on a motorway trip though!
Have you tried resetting the computer and monitoring it for your useage? That reading may have been calculated over thousands of round town only miles.