@Lilypiesmum
You are only technically in arrears due to their incompetence, so complain saying that you will be taking it to the FOS if they don't defer you immediately and cancel them or backdate your deferment.
Setting a collector on you for one month's arrears when it is their fault and you are entitled to have it backdated anyway is also totally out of order and disproportionate thing for them to do. Even if it should be just another part of their company they refer it to.
Add that to the complaint.
Contact the FOS and ask if they will write now on your behalf setting out your complaints.
They say they will here, though sometimes takes a dew calls to get through to an FOS employee that will do it.
www.financial-ombudsman.org.uk/consumer/complaints.htm