Accounts department dont know how to do there job.

I received a letter of demand for a property I am renting and I have paid the rental invoice on the 29th of July. We tried to email them the POP and it bounces back. This morning we have been calling to speak to the accounts department to sort out this issue. The accounts department has still not come back to me. I have lived here for about 3 years now and I am very ****ed off. Firstly they still have not come back to me about the damp in my apartment and on the invoice I dont see my deposit or interest earned on the deposit which by law should be put in an account. I want an apology because if they did your job correctly we wont be here right now. Then the amount on the letter of demand is much more, why is that if I have paid my rent and the invoice I received the amount is alot less.