OK, thanks again Eloy. I will need to find a suitable slot in my timetable to sit and play with the manual backups so it may have to wait a few days.
Please to hear the problems are 'normal'.
We are using Windows - so external zip does not seem to be an option.
I also have a feeling that I switched off error reporting in php.ini - so I'll have to see whether debug on will show any more than now.
We have not knowingly changed any user image manually (not even sure I would know how) - but we do have a number of users who have changed profiles to include scripts (now filtered out) remove names etc. Their underlying user id is still there but, for example, name fields are blank. However there are enough of these pests to show in more than the one course only.
Thanks for your patience and help - I'll get back to you as soon as I can.