I have seen a couple of solitary bees recently (and managed to get photos) and wondered what they are. The first one I saw yesterday (7 June 2012) in our garden in Sandy, Bedfordshire. It looked more white in real life but slightly more reddish when I looked at the photos on the computer. (http://www.flickr.com/photos/orangeaurochs/7346401864/in/photostream) We had red mason bees in our garden (identified here by bombuslucorum http://www.nhm.ac.uk/natureplus/message/18835#18835) but I don't think this is the same thing. I haven't seen a lot of these white ones around. It is on what I think is comfrey.