I have uploaded a link to my MIDI transcription of Simpson's Trio for Clarinet, Cello and Piano to Downloads.
This work was written in 1967, the year before the well-known Quintet for Clarinet and Strings. It is completely different from that work, although like the later work it has enormous energy and passages of serious beauty.
Judging from the symphonies (which, to be honest, are the only works which I'm acquainted with so far), Simpson was an outstanding talent and is nowadays incredibly neglected. I can only begin to imagine the trouble you have taken over this transcription - very many thanks for this!
:)