On Wednesday, 10 December 2014 14:24:10 UTC, Stephen Davenport wrote:

It makes a kind of sense because mid Dec-Jan regimes are often linked, so a cold block, for example, in mid December would more often than not also be January's characteristic.

This is what I was thinking. Cold air in the UK in December, especially prolonged cold suggest it's come from the East which in itself implies that the "East" has cooled down quickly and provide a source of cold and I'd imagine that a colder East with an existing high pressure/cold pool might be prone to more stagnation during the remainder of the winter.

Richard