Mac OSX Snow Leopard, BT HomeHub 2 and FTP Server???

This phenomenon is covered in this wiki guide and this Q&A.
In brief: Change the administration port for your router, or use a different port for FTP than 80.

However, if you get to the router's admin page, it looks like as if you don't use a regular FTP client, but you try to use the reduced browser built-in FTP functionality without specifying the protocol. If you work with a browser, you must use ftp://yourname.dyndns.org/ or ftp://yourname.dyndns.org:portnbr/, if different from port 21.

If you don't want to explicitly specify the protocol and port, you could use a WebHop Redirection.

Also, I would like to discourage you using FTP. This is no longer state of the art, insecure and complicated to handle with firewalls. Better use a HTTPS or SSH/SFTP based solution.