Oddly, the form is now submitting, but it isn't submitting and retaining the web address… what I am getting is mailing-list-thanks.php rather than website.com/mailing-list-thanks.php. Basically, it's scrubbing the domain name and giving me a server not found error.
Why is this?
For the time being I have overcome this by adding the full web address to the UE code on the $GoToPage.