Done nearly 6000 miles in my GT TDi 170 since August '13. My trip computer says I've averaged 44 mpg.
My best and worst mileage from a tank are 47.6mpg down to 37.9mpg. All depends on your style of driving, I do completely town driving 5 days a week, and mostly motorway at the weekend.
Best miles to a tank - 517 miles.
(no cruise control fitted (still). That would reduce my heavy-foot on the motorways. ß