Mr Savage:
With the obvious oil being more viscous when cold (not massively more with newer fully synthetic oils), drier air (that summer humidity helps a lot on the bang expansion) and extra use of lights, heating etc in the winter being a common winter mpg killer to both cars, I knew there'd be an effect. I was under the impression though that the difference between summer and winter mpg was noticeably greater with a diesel than a petrol (maybe 20% diesel vs 10% dip with the petrol), primarily due to the volatility of the petrol meaning it ignites well at all temperature ranges.
My GTD is certainly looking about 20% thirstier on like-for-like journeys right now vs what it was doing in the summer. It seems far more temperature sensitive than any of my previous TDIs - an actual 44mpg around the doors is about the worse I ever saw in my Scirocco at slightly sub-zero temps. I'm seeing about 41mpg now at +5C, so there's always a chance it could get worse.
I wouldn't sweat a 4-5mpg dip in the R. I suppose a 4mpg dip in your GTI is noticeable but not huge, unlike me being down 7mpg on a short journey and 10mpg on a longer one. You've suffered an 11% drop at the same time i've suffered an 18% drop, that's kind of what I was expecting, for the petrol to be less affected (but still affected) by the winter.