Hi Peter
Long time no sea.
Not to give advice regarding this particular case but just to reiterate what I heard a judge tell your brother once:
Under English Law, (The Copyright etc Act of 1988) Copyright is enshrined with the author until 75 years after his demise. It cannot be sold or given away but it can be licensed for use in a particular way for a defined period of time (not imperpetuity). Therefore, it is risky to use any art supplied by a third party unless you have proof that its copyright has been licensed to that person for onward use and that person will indemnify you against action by the artist.
I thought that as a publisher, it was best you were aware of that.
Incidentally, under British Law, misapropriation of Copyright is akin to misapropriation of the contents of someone's bank account, although matters are usually settled amicably outside the Crown Court!
Best wishes
You are doing a good job, by the way.
JB