very poor and i will never recommend anyone to buy from them

I have an account with brands i took couches, am based in East London and i am not happy with their service. i am paying monthly and getting interest on my account monthly, how is that possible? The balance always does not add up, just called them and ask to speak to the manager and he just said every month when your account is in arrears even thou you paying you will get an interest. I told him i will not pay because that was not said or explained to me and i never signed up for that. He just said he will hand me over to the lawyers of which i do not care about that because i will never be charged interest for an account that am paying monthly. His name is Gert