Old Bruce thanks for you swift response. Yes I am using Master Pages with photo frames and, yes, my frames are named in my layers panel. However, the reversal of the pictures order still happens when I insert a page. Interestingly, after your response I copied the layers from the Master of my original book and pasted them into a new Master on a new Publisher document, inserted new photos and tried again - this time it behaved as you described and, after inserting a page, the pictures moved over in the original order. I attach a screenshot of my Layers Panel for my Master A.
It seems to be an issue with the original book that I created even though the Master is an exact copy of the first effort. I can see no difference between the two documents but they are behaving differently!
I have now started rewriting a new document and inserting the pictures all over again - this time with the years in the correct place from the outset! It will be a long haul but I see no alternative.
Thanks again for your reply.