Just for info - The BT Smart Hub 6 will not accept these special characters:
to be used in the broadband password, and worse, the error message gives no clue to this. The solution is to change the password first using the old Plusnet Hub, both on the server and the hub (to serve as a fall-back).
After a deal of frustration I found the following two threads:
https://community.bt.com/t5/Home-setup-Wi-Fi-network/BT-Smart-Hub-HH6-DNSOmatic-setup/td-p/1702696 (note that I saw no content in my error message)