To help anyone else out with a similar issue i.e. prevent you from tearing your hair out unnecessarily, the problem is with BigPipe's CGN (Carrier grade NAT). As I understand it BigPipe's CGN internally reassigns IP addresses to its network users. In my case, this prevented the dynamic DNS service from correctly being able to access my webserver.
The solution is to request a public IP from Bigpipe - which they very speedily & helpfully provided & all is now working as it should. Roll on IPv6! :)