a bunch of usless theives

I have an account with Hi Fi Corporation, which i have been paying from November, i requested a settlement letter from the accounts department on the 11th of May 2015, which i received, i then proceeded with payment as per the settlement letter, also emailing the proof of payment afterwards, requesting for confirmation of receipt, which i never got back. i then found that my Bank account had still been debited the installment after me paying the settlement. when i called the accounts department back, i was informed that the banking details had changed and i have to firstly wait for them to transfer the funds from the old account to the new, and wait until it reflects on their statement, then do a reversal, which could take 7 to 10 days, in the mean time, the money taken from my account was for other debit orders, which will now bounce and leave my credit ratings in a mess. this is pathetic service. i want this rectified with compensation before legal action is taken.