Sorry - I should have posted back with a solution - at least one that worked for me!
The report to Ancestry Support was fruitless. A single reply saying "Have you tried this?" and a FAQ which I had already found.
In the event, I CLEARED THE CACHE IN MY BROWSER - (which happens to be IE9 64bit). What I hadn't realised is that FTM evidently uses the computer's default browser without expressly telling you. Something must have gone wrong with a cookie or similar. I deleted temporary files, history, cookies, saved passwords and web form information, so not sure which was causing the log jam.
Interested to hear if this fixes similar problems for other users - because it doesn't appear as a possible solution in FTM's FAQ article!!