In reply to Martin1978:
Lady Blue used to have a 1997 1.9 Golf Tdi (90hp) and despite being a severly ill car by the time we got rid use to regularly return 50+mpg, up to 65mpg if driven carefully.
She's now got a 2012 Seat Leon Ecomotive (1.6tdi), which returns 55mpg on her (stop-start) commute, and well over 70mpg on motoways. We've not hit the theoretical 84mpg yet, but we've had mid-70's. If you are looking for both fun to drive and economical the Leon is hard to beat, as slowing down for corners appears optional.
Plus its a Golf-based car.
I used to have an 2005 Octavia 1.9tdi (115hp) 4x4, which used to return 55mpg-60mpg on motorways (and I have a heavy foot).
The 2006 passat (1.9tdi 105hp) used to get 45 (heavy foot)-55 (economical)mpg on motoways. Quite a bit worse than the Octavia despite not being that much bigger, and having a lower power turbo.
I've currently got a 2011 Skoda Superb 4x4 2.0tdi (170hp), and that does about 45 mpg on motorways if seriously tanking it (90-100mph+), 50mpg at normal motorway speeds (70-80mph), 55mpg at 65mph-75mph and on one notable day where I had a puncture and hence was limited to 50mph but had to do a long journey whether I liked the idea of driving on a space saver spare or not returned 70mpg, which is outstanding for such a big car.
The above gives you a range for VAG-based cars, both current and past. Hope helpful. Basically it looks, from my experience and what you say about your driving, that any of the golf-based (Golf/A3/Leon/Octavia) 1.9tdi or 2.0tdi engines (with possible exception of the 170hp version of the 2.0tdi) will meet your criteria, and the 1.6tdi versions will exceed them.