Fixed this problem by adding the following lines into filter_api.php:

$query2 .= " ORDER BY '$c_sort' $c_dir";

Mark Ingram

    # 26/05/04
    # Added this if statement to always show most recently updated bugs at top
    # followed by the most recently added bugs

if ( $c_sort != "last_updated" ) {
$query2 .= ", last_updated DESC, date_submitted DESC";
}
else {
$query2 .= ", date_submitted DESC";
}