Whilst browsing for some tree info I came across the following, which
might be of interest.

"Bean's Trees and Shrubs Hardy in the British Isles" is a very useful
source of information. The International Dendrology Society has made the
text of the four volumes from the 70s and the 1988 supplement freely
available at:

http://www.beanstreesandshrubs.org

An update book, "New Trees: Recent Introductions to Cultivation", was
published in 2009, but is not available online (more info at
http://www.kew.org/publications/newtrees_presspack.pdf for those
interested and who have a spare £99!).

--

Jeff