Thanks @FordGuy61 - refreshing the metadata was one of the first things I tried; I tried again but it makes no difference; but I appreciate your tip about the episode ordering - this worked for me 
Here you can see Ben Foygle in the background of the show
Problem is this:
The effort of doing the Plex Dance is not really justified for this - I can live with it as it is, and I really wanted to change it I would just upload another image and then select that.
But it would be much better if Plex would allow you to deselect the image. The logic for this must be there as you can select images if there is more than one. For some reason, this is disabled when there is only one image.
