All fixed @RobbieMac - very happy.
So the problem was that my brand new account was setup as an Account Manager, not the Account Holder.
I've maked the solution as just that.
Thanks again for taking the time to help - really appreciated.
Stay safe !
Have exactly the same issue and receive the same message. Have tried to resolve through the complaints process but no one seems able to offer a solution. The best BT could do was to offer to credit my account for the amount I would have paid out for a 6 month subscription but this does not address the issue that the offer as advertised should be available. At least nice to know I am not the only one.