I’m still new to WP and not sure what you’re asking me to look for. However, I have solved it!!
Since my last post I have tried a few things and noted that the thumbnail (or medium image) was correctly shown if the product was uploaded by spreadsheet and the images were in the month/year folder, but the thumbnail was not used if the same product is uploaded by spreadsheet and the images were in another folder (images having been previously moved with ‘Media File Manager’ plugin). I checked on my server and the same set of images were in the product folder as were previously in the month/year folder.
I then deactivated ‘Media File Manager’ and used another plugin, ‘Media Library Plus’, to upload the files directly to the product folder. UPC then correctly picks the medium sized image for the main catalogue.
Problem was obviously something within the ‘Media File Manager’ plugin that was not correctly updating the database. I now just have to upload all the files again with the new plugin, but at least UPC is now doing what it is supposed to.
Thank you for your help.