Bad experience

I have set up a debit order for my ackermas account ,which they have been debiting fine until i changed my banking details .I changed banks in december which i also made them aware of in time .Since then my account has not been debited went to a store to report this which they gave me a phone to call the accounts department ,lady that i spoke to spoke over me and did not show any willingness to assist . Than I have received emails last month telling me about FINAL REMINDER - OVERDUE STATUS OF YOUR ACCOUNT! which I responded to and we made payment arrangements and agreed they will be debiting on the 29/03/2018 still they did not.I then sent them an email to let them know that they still havent debited my account they asked me to confirm my banking details which i have don more than 3 times .They now threating to hand me over to collection and ruining my name