Have a good look at the Pedelecs forum, there is plenty of info on there. Here is one to get you started, I think it was stated by someone who is also a member of this forum.

http://www.pedelecs.co.uk/forum/elec...onversion.html

There are other Brompton threads on there as well as discussions about other folding bike conversions. I have been following one about a Mezzo conversion.