Solved! Go to Solution.
Add your existing account when you start the app on the YV box.
If you wanted to have a catch-all with BT supplying the service via your BT bill you'll probably have to cancel your existing subscription and take one out with BT.
http://bt.custhelp.com/app/answers/detail/a_id/51430/~/set-up-and-use-netflix-from-bt
Personally I'd just keep them separate - one extra DD isn't exactly a chore to manage.