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.