In a bid to work out what's causing my problems with FB and Messenger, I have installed Firefox - albeit an older version, as my ancient OS doesn't support the latest one.
It is v e r y slooooooooooow, but it does seem to work. And something is better than nothing.
Which would indicate that the problem appears to be some kind of software miscommunication between FB and Chrome. I was using the latest version of Chrome, but went back to a previous version the other day to see if it would make a difference (it didn't).
It's frustrating, but what can I do...