You can accurise the Trip Computer.
Work out the actual MPG figure from a full tank
Number of litres added / 4.54 = number of gallons used
Actual MPG = distance on full tank / gallons used
Go into VCDS/VAGCOM, open controller 17, Instruments
Click advanced functions, Adaptation box 10
Scroll up to channel 3
Standard default value = 100%. This can be altered withing the range 85% to 115%
Work out new value this way:
New % figure = (old % figure x MPG figure shown on dash) / calculated actual MPG figure
I have done both my cars and it's pretty much spot on now
Disclaimer: Neither I nor MK5GolfGTi will be held responsible if anything goes wrong. If you do this procedure you do so at your own risk!