Just to close this one off, I got the following reply from Webfaction and the problem has been resolved:
"I created a manual redirect app for your sites HTTP record called 'dth_2018_redirect'. As per,
https://docs.webfaction.com/software/static.html?highlight=http%2520to%2520https#redirecting-from-http-to-https
And the site doesn't seem to be stuck anymore in the redirect loop."
And I can confirm that the issue has been solved.