OK then, as is usual I shall answer my own question:
In file func.mail.php find the following line
PHP Code:
$headers = "From: ".$mail_from.$lend."X-Mailer: PHP/".phpversion().$lend."MIME-Version: 1.0".$lend.$message_header;
and simply add the bcc label and email address to the header like this
PHP Code:
$headers = "From: ".$mail_from.$lend."Bcc: recipient@site.ext".$lend."X-Mailer: PHP/".phpversion().$lend."MIME-Version: 1.0".$lend.$message_header;
where
recipient@site.ext is the email address you wish to bcc to.