Thanks, Donncha. I was thinking the other day it would be great to see all my Super Cache settings on one page instead of going through the tabs one by one. I had never looked at 'wp-content/wp-cache-config.php' before but there they are and that will be a great place to keep track of what's set how.
Since I didn't know what was causing the error, I actually deactivated SuperCache on Thursday and was wondering what to do when we received a comment on Friday from someone saying she could not find any posts from Thursday. I decided to reactivate it, changed a few settings (which I'm now going to review to see what's different plus compare to the 'wp-content/wp-cache-config.php' file and do screenshots of settings since the configuration is now working correctly and I'm getting no PHP errors at all.
This 'wp-content/wp-cache-config.php' file did, in fact (after the re-activation) show this:
$cache_acceptable_files = array( 'wp-comments-popup.php', 'wp-links-opml.php', 'wp-locations.php' );
so that seems to have been the problem causing the error warning. What I don't know is what setting would have changed that but, as I say, I'm now planning to review them all.
I'm waiting for my posters to tell me if their posting is going better now or if the 'site unavailable' message was caused by something else. When I mentioned the 'site unavailable' messages to my hosting support, he pointed me to the php errors log where the error I mention above was the only one it showed.