Thats the head office. It may well get 'misplaced' in the Ntl internal mail.
Depending on where you are in the country it will either be Coventry or another office (Teeside possibly?) that will handle the disconnection.
EDIT: This is the Coventry address. They might be handling all the disconnections now. It's only been 6 weeks since I left so plenty of time for a few more re-organizations
ntl Customer Relations
PO Box 3987
Coventry
CV1 9AS