Ah well! I was supposed to get something - permission to use the painting was sought on behalf of Martin Bowman by my mate Derek (Norfolk man living in Copenhagen) who'd organised the 50th anniversary memorial of the Shell House raid, which is why, and how, the painting was produced in the first place. Derek is a friend / associate of MB, and I left it with him. However, I then went through a Divorce around the about the time the book was being published, moved house etc., and I guess it just got forgotten in the physical and emotional upheaval.
As, far as I know, there isn't even a credit either, unless it's on the dust cover somewhere (my copy was downloaded - didn't get my signed copy either!). I suppose I could chase it up, but it seems a bit churlish, especially after 14 years.