Copper resistance drops by 0.4% with every degree C, however hasn't the early morning temperature risen by 5 degrees since the cloud cover arrived? We've been seeing -5C in Herts most nights for the previous week, but only just freezing yesterday and today.

My guess is that the 2-3inch blanket of snow on the ground helps with shielding RF interference from underground cables.

The reduction in road traffic might also have helped. I saw a very significant line sync increase (an extra 1Mbps) over the Xmas holidays, so perhaps there is a net reduction in RF background noise when people aren't at work.

I think that's more likely. At the depth of most BT ducts, you're not likely to see much of a temperature change anyway.