Possibly I have another data point for this thread. I've just switched to use the Opera browser to try to bypass the problem, it worked! Initially Opera did show the iframes as expected. Ahh but that would be too easy to be the end of the story...
so on my snhiny new Opera browser, I signed in to Lastpass and also copied my bookmarks across by copying the bookmarks file from Chrome's data store and pasting the same fie to the opera data folder (basically Chrome and opera run on the same codebase). Having done these 2 things, the iframes stopped showing again.
Lastpass problem?
Bookmarks problem?
just plain old bad timing?
Anyway - there it is. still not solved but a bit more info added to the pile