I found a way to resolve the issue below, by using the automatic software installer to create a clone of the installation in another folder which install was completely successful. Then having removed the original installation, I made a second clone in the original folder, also successful.
I needed to reinstall phpList from scratch using an existing SQL database. I used the automatic software installation app provided by the webhost. This reported a failed install arising I think from a failure when addressing the existing database during installation.
After installation however, phpList was working and my subscriber lists and my templates etc. were there.
However, the subscribe page (accessible by the public) was being redirected to 404 because of this in the .htaccess file in the lists folder:-
Redirect 404 /lists/index.php
Judging from the comment lines the installer would have removed these lines if the install had been noted as successful.
Removing the redirect in the .htaaccess file restored access to the subscribe page.
I have auto-update switched on so I’m a bit fearful that the redirect line might be reinstated by an update.
Does anyone know if the automatic update does reinstate the .htaccess file in the public folder?