Have just tried from a BT Retail Residential and works fine from 86.155.x.x address, so it seems to only affect BT Retail bsuiness customers so far.
As requested, updated on BT business forums:
Ray - I assume you are using a BT residential service? What are the first 2 numbers of you public IP address?
From a reply to your post over there.
"The only thing that I can think of is that Gradwell are possibly blocking a certain range of IP addresses."
That is a possibility.
It seems the BT/Gradwell firewall issues have been resolved.
It would appear that 81.x.x.x networks (mostly BT business??) were denied access to gradwell networks, however I have been informed that this is now resolved. Ray could access OK because he was on a BT 86.x.x.x network.