Every time i get into the car the MPG has reset any ideas?

Sounds like you are looking at trip computer 1, which resets after 2 hours.  Trip computer 2 needs a proper reset and will show the longer term averages between each reset. You flick between the two which the switch on the bottom of the stalk (or possibly the wheel in newer cars?).

There are short and long term settings. Are you looking at the right one?

Also, the short term ones have a set time before they reset but I'm not sure how long that tine is.

Edit: Just pipped to the post with the answer!

Every time i get into the car the MPG has reset any ideas?

press the button under the RH stalk.  display should change to have a small 2 in the top right.  This is the long term average but it resets every 99 hrs of vehicle running.

 

It's in the manual.