Thanks for replying. It’s a hosting service. We have several wordpress sires on the same server. looking at one of the sites which is working on SSH, and running ls -Z, it shows the correctly running site has all the files as being in the group psacln, but on the broken site, a couple are psacln and the rest are root.
psacln seems to be a standard group set up by Plesk. Presumably, I need to add the files from the broken site to pscln too, but I wouldn’t know how to begin in doing so?