Interest to be paid due to poor service

I had an account with Wonga, In dec I asked Nimble to give me a settlement letter, I did this via your WHATS UP, and via email, and phone calls. I did not receive it, I asked In January again, in February again, and now you have charged me interest. I have still not received a letter from you and am held liable for your interest. I will not be paying your interest as it is lacking from your side.