If you have a look at one of my previous posts, says to leave the payee blank so you have done the right thing, don't worry. HFO have 12 working days to respond plus 2 to cover postage, so Monday is Day 1. You can then send the Account in Dispute letter if they do not supply a proper agreement.
Again, it is important not to speak to them on the phone unless you can record it ,'in writing only' and be as rude as you like.
I think is imperative to phone BC and get to the bottom of where this has come from, who owns it and the last payment date, if it is yours.
Also look at making some complaints to OFT and sending them copies of the letters and attachments. This is for the good of everyone who is threatened by these.
You do not need to show us the HMCS attachments, I know what they are and they have no right to be sending this out.