Credit Controllers bill incorrectly

I changed my banking details with ADT in January, I received numerous emails from their contact centre confirming the change was actioned. When my first debit order went off the wrong account, I again emailed the contact centre was again notified it has been changed. so for the past 6 months I never know from which account the debit order will be taken and in the process 3 debit orders bounced. I have enquired with them my outstanding balance but received no feedback. Monday I receive a final letter of demand from Ms Gamieldien regarding my account. I immediately forward her all the emails I received from ADT and explained the situation, she just reverts saying I have 7 days to pay. She sent me numerous emails, all with different balances, and on her last chain email she changed the reference number, so I rep**** with the new reference, this morning I find out this reference belongs to another customer, and I am being sent this invoices as well. this is ridiculous. I phone her to sort it out and she shouts at me for using the incorrect reference - WHICH SHE GAVE ME!!! I am finished. I want to cancel their services and sort out this account asap.