Thank you Gentlemen for the confirmations, information and comments.
I now know who to contact to sort this out, as it is surely plugin related and not Theme or WP-core related.
@Scrambler Yes, you are right, a 100×100 featured thumbnail image ought not to have smaller requirements…. But 2 of my 14 have links to 250×250 and 300×300 images which do not exist and whose URLs return a 404.
A little coding issue there I think 🙂
Regards and thanks,
Angus