Thanks for the suggestion Keith - seems like a detective job just trying to find out what's going on.
Your link delivered the following error.
"Sorry - You must be connected to BT Broadband or the BT network to use this website."
Unfortunately that sums up the problem.
Just had a call from BT - after 27 hours they have spoken to BT Openreach, who have now accepted the fault. This could have taken minutes from when I first reported the fault yesterday.
No indication of when it will be fixed though.