...in fact it turns out that the missing thumbnails issue was due to the files being accessed via a symbolic link.
Viewing the images using the real link I think creates the windows thumbnails, although they do not display, even on a refresh: need to close and open MC and then the TN are all available on both real and symbolic path.
Viewing the images using the real link I think creates the windows thumbnails, although they do not display, even on a refresh: need to close and open MC and then the TN are all available on both real and symbolic path.