It did eventually get it to display a sensible error message which said line 29 in mailer.inc.php was calling an invalid function
$subject = htmlspecialchars_decode($subject, ENT_QUOTES);
replacing htmlspecialchars_decode with html_entity_decode seems ...
$subject = htmlspecialchars_decode($subject, ENT_QUOTES);
replacing htmlspecialchars_decode with html_entity_decode seems ...