Bumblebee or Humblebee ?

I think this is a Buff-tailed Humblebee (Bombus terrestris) but I could be wrong. The slightly larger White-tailed Bumblebee (Bombus lucorum) looks very similar. Perhaps you know the difference and can put me right? Anyway, there were several of them busy getting nectar and covering themselves with pollen in the Charlton Down Nature Reserve last evening.