Hi Bozz,
If you are referring to the digital miles display on the MFD display, then that is the estimated miles to the next fuel stop at your current mpg usage. So the needle might be at half way, but for example, if you are driving everywhere in 1st at 4,000 revs then the fuel aint going to last long...thus the estimated mileage will go down quickly.
if you drive more eco like, changing gear at the right time, not revving the boots off the thing, then the estimated mileage will not go down so quick. As well as my petrol GTi, I have a 140ps 2.0 TDI Tiguan and can actually make the estimated miles to the next fill up go up on that by driving in a boring manner!
Hope that makes sense.