SagePay php custom server integration, notificationURL issue - server

Question

I am trying to get a custom Sagepay server integration to work.
I am managing to send the initial Curl request to SAGE, get the "NextURL" from Sage and redirect.
From the Sage card selection page I am cancelling (clicking cancel in the bottom left) and getting the following error:
enter image description here
My test can be found at:
https://doors2size.co.uk/sage/server/
Click the "GO TO SAGE" link.
The NotificationURL is:
https://doors2size.co.uk/sage/server/listener.php
The code adding the NotificationURL to the initial Curl request is:
$str_post .= '&NotificationURL=https://doors2size.co.uk/sage/server/listener.php';
To keep things simple for initial testing I have not put any conditional statements on the listener file, it just returns fixed Status, RedirectURL and StatusDetail.
You can follow the link to the listener file (notification URL) and see what it is returning.
The code for the listener file is:
<?php
ob_flush();
header("Content-type: text/plain");
$redirect_url="Status=OK" . chr(13) . chr(10) ;
$redirect_url.="RedirectURL=https://doors2size.co.uk/cart/" . chr(13) . chr(10) ;
$redirect_url.="StatusDetail=".$_POST['Status']." response success." . chr(13) . chr(10) ;
echo $redirect_url;
?>
The listener file does not even seem to be getting called from SagePay.
Any help with what is going wrong here would be greatly appreciated.
Thank