It takes a couple of tank fulls and monitoring to get an accurate figure, i wouldn't go by the first tank. I found that with a full tank, the needle will drop quite quickly but slow down, its not consistant. With each tank full my driving varies from cruise about to rag the **** out of the old girl. Keep the revs down if you're skint, but otherwise, enjoy. Let me know how it goes after a couple of tanks. Fill her up, reset the trip and drive it how you want. if it takes ?30 and you get 250 miles out of her thats like 12p a mile. In my opinion, thats good for a gti, especially a cab. People who get over 300 miles can't be driving their cars that hard. It also helps to keep the revs low untill the oil has reached a good operating temp.
shaun