I just remembered...BT have some test login accounts that help you verify where the problem might be. Here's an extract from another site:
BT have a test login account that could be used to identify if a particular problem is with the hardware you have installed or with our broadband service. If you can manage to get a connection sometimes but are experiencing intermittent problems, you can test the connection using this alternative login.
Username = bt_test@startup_domain
Password = blank
Once you have authenticated, you can browse to the web site,
http://www.bt.net/digitaldemo which consists of a single image.
The important bit:
If you can connect with this login you can reasonably assume that the problems are not caused by the broadband hardware but are more likely to be a configuration issue (e.g. username/password) or possibly a service issue.
Source
Let me know it that helps
amofw