Thank you for replying and getting back to me again.
Well that is what I decided to do. Kind of.
I prefer would prefer to use a real mirror server but I have now temporarily created a copy of the whole site: data, scripts and database. "Replicating" it on windows would not be possible either since I cannot write to the C:\ drive in the machine in the office. Also it really isn't the same and may lead me astray.
When I ran the upgrade it gave me the "upgrade_log already exists" error. I was not expecting that which means I must have missed a step in the restore.
I tried the restored 1.9 site and it failed to load the home page (only gets to '</head>') but I think this is because of our customised code plus the fact that it had been moved to a new directory. I will replace it with 1.9.19+ code, downloaded afresh.
At this point it would be helpful to know whether I can set the theme without using the web app?
I think the theme will be set to something a clean 1.9 version would not recognise. If it doesn't find the theme files does it drop back to standard white?
I am going on vacation on thursday and won't be back till 11th August and I have been told to avoid doing this in that week.
By the way I did not just start this at the end of last week. We had some real trouble getting a verified (checksummed) backup onto our windows network and had to use a usb hard drive in the end. That process took until last Wednesday.
Now I really can see why they say "considering not upgrading at all" ie doing an install and then restoring the courses. I know for fact I won't be given the time to do that, though.