I have finally got this to work. It was a username & password issue.
For anyone having this same problem, what worked for me was your normal assigned [email protected] user but with btbroadband.com instead of btinternet.com
Then your actual assigned password aswell, not just 0 or <blank>.
This is different to what I was told from BT support but then I was surprised they said anything as it was a 3rd party router they don't support.
Hope this helps someone.