Nick, right after lots of stress over the weekend and several hours today we have Worldpay working. I am not sure if I can submit anything updated yet as I didn't work on the troubleshooting. However, in short, the zenID was generating an & in the ID which would of course break it. Some code has been added to check this before passing it and ensure the chars do not encode into an &. So when the session was coming back the ID was different so got forwarded to the page you were seeing as a 302 as the session didn’t exist under that id. - That was received from the guy who helped me.