Sean; I have a book that covers all variants of the 980/981 and every trl that was used with the combination. It covers wartime and post war configurations:
Title - Diamond T 980/981
Author(s) Les Freathy,Robin Pearson
Published 2001 by Roundoak Publishing
ISBN 1-871565-36-7
email:
roundoak@netcomuk.co.uk
or :
www.nynehead-books.co.uk
This is an excellent reference source with dozens of both colour and B&W photos.
Hope this helps.
Cheers, Bill
(ps, if I ever win the lottery I'll own one)