Nature is always changing. The population of some species are rapidly declining (the cuckoo for example) whereas as others are expanding (like the little egret). We have new species colonising our countryside which are a concern (like the harlequin ladybird) and others that are a welcome addition to our fauna. The tree bumble-bee (Bombus hypnorum) is certainly one of the later.
Bombus hypnorum has steadily expanded its range across Europe in recent years and was first recorded in the United Kingdom in 2000 and so perhaps we should call this the millennium bee? It seems to have an affinity to urban areas and to woodland habitat of all kinds and it is expected to become widespread and common across the whole country in time which is a refreshing change as some species of bumble-bee are in serious decline in the United Kingdom.
I saw these for the first time in Puddletown Forest (May 2012), an area I would not rate highly for wildlife interest and yet there were several of them so they seem well established there. I have no dubt I will see them in other places as time moves on.