The Bumble Bee Conservation Trust website has a wealth of fascinating information about the life cycle of bumble bees.
I was correct in thinking this was a large queen bee hibernating for the winter. It seems that queens can go into hibernation for up to nine months of the year, which is in fact three quarters of their lifecycle.
They dig a hole for themselves about 10cm down in well drained soil. Unbelievably, they can easily survive temperatures as low as -19C.
Emerging in spring, they begin to establish new nests. Sadly, the next year she will die, but only after she has produced several broods of young including new queens to continue the 'royal' bloodline.