We have put together a short survey to capture your feedback in terms of what BT can do when you experience home Wi-Fi issues. We would appreciate it if you could take a couple of minutes to complete a short survey. Read more from here, Fix my home Wi-Fi
You could try this: from BT Code of Practice
I would stop any DD or other automated payment process; telling them that I am not cancelling, merely suspending, auto payments whist BT "check everything"
Is that really a sensible suggestion though?
I know it should be, that according to BTs own code of practice this is what you should do, but given BT's track record I'm sure it would be safer to chase a refund than to cancel a direct debit, risk them getting the whole situation back to front, and end up with BT cancelling your service because you've not made payments, thus making you unable to contact them easily, because you have no service.