Ok firstly apologies for not nailing it before .. excuse is that I am having to get up at what I consider stupid o'clock, so was heading off for meal and an early bed when I saw that it was broken. I did a rushed job and hoped for the best. As it is I have now fixed it .. well hopefully ...and I know what has caused it which is good.
1) What appears to have happened is that our hosting providers have done a minor software upgrade neglecting (of course) to let anyone know.
2) This caused a couple of lines of old code on our admin site to fail which meant that the whole admin site crashed. I literally removed two characters ("&''s") from the code and it was back. FIXED! that is what I did last night.
3) The software upgrade wrote two NEW files with our default settings in. Luckily for all concerned it made back up copies of the old ones. So all the tinkering to allow large files to be uploaded .. stopping most timeouts etc was gone!! Hence it not working .. anything over 10mb crashed it.
I have been through old and new and have upped limits in our new config files to match those in the old one ... chances are that I missed summat (there are over 1,000 lines in the file) but uploads work ok now!
