FNB/Wesbank has been charging me for insurance and claiming I don't have insurance despite the fact that I do

I have a vehicle financed with FNB/Wesbank. The vehicle is now in its 5th year of Finance with them. I am in the process of selling the vehicle and thus I requested a full and final settlement letter from the bank. To my surprise I noticed that the settlement value requested from FNB/Wesbank was way more than what I had expected. After inquiring with FNB/Wesbank I was told that about 12 months ago FNB/Wesbank had decided that I did not have comprehensive car insurance and they began to charge me an extra R800 every month. FNB/Wesbank did not inquire or ask me if I have comprehensive car insurance and they also did not inform me that they are now charing me an extra R800 for "insurance". I do have comprehensive car in insurance and have been paying this insurance, without a single payment outstanding, for the past 5 years. To add insult to injury they also started to charge me penalties because I was not paying them the R800 they had decide to charge me. I have contacted FNB/Wesbank's call centre and I have been pushed around from person to person, I have provide them with all the proof they need to show that I do have full comprehensive car insurance. The big problem is I have a buyer who wants to now by my car and FNB/Wesbank is trying to bully me into signing the settlement letter as is. They are saying that another department will sort this issue out later. How can they ask me to accept this current settlement? As soon as I accept the settlement letter the matter is closed. I find their behaviour extremely ********* and questionable.