Bumblebee in December!

I was very surprised to see the above Bumblebee gathering nectar from my Potato plant Solanum, which remarkably is in flower in December (picture taken on 20/12/09).

Due to its size, I think it must be a worker of either a white- or buff-tailed bumblebee Bombus lucorum or Bombus terrestris. If anyone has any ideas on the species ID then please let me know. The temperature on the 20th was barely above freezing and I cannot remember ever seeing a bumblebee when it has been that cold.

According to the BBC Wildlife magazine for January, there are some nests that are active throughout the winter, which must account for my observation. The magazine does point out that in warmer weather, the larger queens of the two mentioned species will also be prospecting for new nest sites. With the current ongoing freeze and 35cm depth of snow on the ground, it could be sometime before the first queen turns up in my garden.