"What happens to bats in winter?
Because there are few insects to be found in this country during winter, British bats hibernate. They
find a cool place, free from disturbance, sometimes a cave or disused tunnel, and tuck themselves
away, often in a crack or crevice. Their heartbeat and breathing slow down and their temperature
drops, so they use very little energy. Sometimes you may see a bat flying during the winter as they do
wake occasionally, to move to a new site or look for food or water."
From...(pdf)
http://www.bats.org.uk/publications_download.../British_Bats_English_3_09.pdf