As I feared mysqli was the answer

Having banged my head against a brick wall trying to call mysql stored procedures and actually get the results from the output parameters in php we finally installed php5 and mysqli on the server.

$sql="call insert_account(
'$SQLEmailAddress','$SQLPassword','$SQLFirstName',@retval);
select @retval as return_string;";
if (mysqli_multi_query($link,$sql)) {
do {
    if ($result = mysqli_store_result($link)) {
        $row = mysqli_fetch_row($result);
        $ResultString=$row["return_string"];
        if ($ResultString!="S") {
		echo "There is already an account for this email address. ";
	}
	else {
		echo "Account created successfully. You will shortly be receiving an email
			with a link to confirm your registration.";
		include("confirmationemail.php");
      }
      mysqli_free_result($result);
    }
  } while (mysqli_next_result($link));
}
mysqli_close($link);

Job done!