Hello Richard. I think you correctly identified the problem in your first post: your site is protected and so Moodle's server can't reach it. I am not sure there is a workaround.
Thanks for your reply, but I would really like to get the site registered.
Is there anything you can do? The URL is quite simple, everyone can view the site here:
It would be strange that you can't register a site that authenticates with SAML.