Update: Problems Resolved. No idea what that was all about..
** Issue **
Unable to edit any posts or pages: no text appears. Post by email working fine.
** Detail **
The site automatically updated to the latest version of WordPress last night, but it appears to have messed up the editor.Looks like I can still post from my phone, so that will have to do for now.
** Update **
After spending an afternoon trying to get the editor working again, I gave up and just used my remote-posting system. No amount of faffing about with plugins or security made any difference.
So I logged in this morning (Monday) expecting more hassle, but found that everything was working again. So it looks like it was a hosting or cdn problem. I suspect that it was either a dos attack (again), or just a LOT of traffic.. Anyway, it’s all working again now.