I had a quick look around and found this that may help GSS :
If your ISP is being difficult raise a complaint :
http://www.oftel.gov.ukand you can talk to them on the phone takes 5 minutes,.
2 days later tiscali call me back as a result of the oftel complaint and as if by magic are now able to give me the CBUK number associated with my line,.
You could try that GSS,have you had your account longer than the minimum contract period as they may be allowed to withold the number until then or until you have paid the cancellation fee for early cancellation
For anyone who doesnt know what we are talking about
What is a CBUK number?
All ADSL accounts are assigned a unique CBUK number that can be referenced by BT. When you open an account,BT will give this number to your provider. You will need to know your CBUK number before you can migrate an account to another provider.
Hope that this helps :doggie: