Beyond Frustrated

I made a payment arrangement with Lime Loans which I have been sticking to, a payment has not been skipped or missed since the payment arrangement was made, however even though they didn't even bother to notify me, I've now discovered that the account has been handed over and the company now handling the account has sent me a statement showing that double the amount which I have already paid is still owing due to interest and fees. What is the point of making a payment arrangement if it's not going to apply on their side. Trying to fight them against this is like trying to argue with a brick wall.