Unless you're going heavy on the aircon when it warms up a bit, or heavy on the heating (rear windscreen, wing mirrors, heated seats all on together with aircon for demisting), the mpg seems to shoot up above 8C ambient temp on the GTD like no other TDI I have previously owned. Modest use of the preheater to maintain 16C in the cabin at a low fan speed without aircon seems to have no measurable mpg penalty for me when ambient temp is over 5C.
At 12C on Thursday afternoon, coming home from work, I drove the car hard on my 13 mile commute and still got 49mpg, that might've been 38mpg a few weeks ago at 2C on the morning commute.
The cold seems to kill GTD mpg the way it is set up for EU6 compliance (less air in and compressed makes for less heat generated in the compression cycle, poorer mpg for longer while warming up, as well as less generated NOx - probably why the MK7 TDIs take an age to warm up), make the most of the warmer weather.