Theft and incompetence

I have been trying to close my account with and request a close of account letter but there has been no action they have been adding on amounts saying I am liable for payment even though I've requested they close the account. It's been months now and they are not getting back to me. I've called and emailed and the RCS group are showing an extreme load on of incompetence and theft! I've also requested my full statement from the inception of the account and each time I am promised it I get the current month's statement and balance. Please help I urgently need this letter of closure.