Neil, I suppose you've already found these sites but I thought I'd post them so you know there is still info out there on them.
Engine data: http://www.oldengine.org/members/diesel/ListerData/spec.htm
The next two links are from tha National Archives:
http://www.nationalarchives.gov.uk/a2a/records.aspx?cat=007-drake&cid=-1#-1
http://www.nationalarchives.gov.uk/a2a/records.aspx?cat=007-drake&cid=0#0
It looks like there is a lot of documentation there if you can figure out how to access it. I couldn't.