@vtxyzzy Thanks - the SQL query threw up 10 records, and gave the user ids. Only one of those user ids appear in wp_users - my own admin record. This seems to give a complete explanation of what I see in the Users list in the WP control panel.
I made a backup of the entire site, deleted records in wp_usermeta for those users, excluding my own, and the Users list now looks normal.
One thing I don't understand is that I re-ran the query expecting to see just my record, and it showed 0 records.
So I browsed the table and my record is still there, still as administrator - it is still present in wp_users and shows in the WP control panel, so it seems OK.
@esmi Thanks. I'll look through those links. As I had done a full restore from backup, updated the version and then hardened with a variety of plugins, I thought the installation had to be clean. Obviously not!
Vic