What a good question!
I wrote my own plugin to do the fancy rearrangement of images on the gallery page (try resizing the browser at
lizclay.co.uk/gallery) and some other stuff. The plugin only loads css and javascript - but for some reason I made some "simple" changes to the html in the function onContentPrepare. Maybe I was doing a test (see attached).
Anyway, I commented out that line and the plugin now works.
Thanks for the guidance.
Ben