Excellent!
Using either the Internal, or the Windows, alternatives, the thumbnails appear.
Initially, there appeared to be a lack of consistency in behaviour as some heavily populated folders displayed all images after a few seconds, whilst other, less populated, files simply displayed the default icon.
I then tried changing the Number of threads setting from Auto to 10. This seems to help.
Unless there are any unexpected issues I am using the Windows option, which, despite implying Win 7+, seems fine, so far. I have not had time to exhaustively test for consistency and repeatability but at the moment everything does seem to be well-behaved.
Some folders partially populate with images, but by reverting to a List view and then back to a Thumbnail view the missing images get created.
Thanks for your courteous response to yet another query.
The more I explore this software, the more impressive it becomes!