Well, I was guessing when I wrote the post above, but a book by Tony on the Class 15/16s has now appeared on both the Pen & Sword and Book Depository websites, for publication in October.
Book Depository also advertise one on the Class 14s; publication in 2022.
I'll be ordering these two as I've found the previous three very interesting. I'm still reading and enjoying the one on the Claytons. In the past most of what I've read on the NBL class 21/29s, the Metrovicks and the Claytons could be summed up as "they were unreliable". I hadn't previously seen any author give an explanation of why this was, and what was done to try and fix things, as Tony does.