Just got the email from BT referring us to the new changes in terms & conditions as of Dec 2nd 2008.
Ending the service
8. When you end your broadband service outside your minimum period and do not request and use a migration access code to move to another service provider, you will have to pay a cease charge of £18.51 (from 2nd December 2008). You will not have to pay this charge in the event you are moving home and we are unable to provide the service at the new address.