THANKS to all who have so kindly replied!

I've ended up with the following, which shows fine in IE 6, even though it's somewhat short:

use CGI;
$query = CGI::new();
print $query->header(-type=>'text/html',
                     -status=>'403 Forbidden');
print $query->start_html('Acesss Forbidden!');
print $query->h1('Access Forbidden!');
print $query->p("Access to this resource is forbidden.");
print $query->h2('Error 403');
print $query->end_html();
[download]

As I have NO idea of exactly where to find default one from Apache on my host, and I may not have access to it via FTP anyway, so this will have to do...