by the Beezelbarb.
I should have left well enough alone. I had a great working version - we just didn't want it to display the moodle folder in the URL. But apparently it's not best practice to install at the root of the domain, even though the documentation says this is an option? Because once I did that, it broke both sites.
I will try another install as you have suggested, but I'm not confident now that I haven't screwed up the permissions for the entire httpdocs folder.