B hypnorum was first reported in the UK in 2001, and has now established itself over most of the country. They are distinctive little bees, with white tails and a rusty brown thorax. They regularly form colonies above the ground, using various holes in trees, buildings, and often use bird nest boxes. An useful information sheet can be found here: http://hymettus.org.uk/downloads/Info_sheets_2010/Bombus_hypnorum_infosheet.pdf
It is the first time I have seen these bees in Swaffham, but have since also identified them down in Suffolk.