Sabinooo’s i/p prompted me to take another go. I got ftp access, retrieved the .htaccess file, and its contents looked like this:
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ – [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress
Comparing it to Sabinoo’s suggesteed code, I saw that I needed to remove the first RewriteRule command. I did so, uploaded the changed file, and the symptom has remained :-(.