I'd guess there was something very odd going on. Some of the text in the docx file was copied from a Pages document. Bizarrely enough, there seemed to be an 'invisible' image carried over with it! What I mean by that, is the thumb nail for the docx file showed a picture (that WAS in the Pages file) but when the DocX file was opened - no image to be seen! Also, my number paragraphs were strangely formatted too. So, I'd guess that there was some other rogue formatting hiding in the docx file along with that image - all carried over from the Pages doc.
To fix it I cut/pasted all my text from the original docx file I wanted to use, into a NEW docx file, saved then put it into Publisher - and, bingo, it all worked after that. No more hidden images or weird formatting. I think it was swapping between word processing apps on my Mac that threw a spanner in the works.
So, all sorted now (I hope!!)