CSA keeps taking more and more from my payslip

Through the sheer awkwardness of my ex she refused access to my son and falsely accused me of assault which resulted in me being arrested without charge.

Due to the lack of access to my son I refused to pay CSA or have any contact with them until access was granted. Approximately 7 months later access was granted and I contacted the CSA about setting up payments and catching up with the arrears. Unfortunately, i was then signed off by the doctor for 7 weeks, during which I only received SSP. I informed the CSA of this and they advised me to send my payslips proving this. i did this and heard nothing more. I presumed everything was ok until i returned to work and received my normal payslip, they had taken extra payments for CSA, as it was only £20 I thought it wasn’t worth worrying about. However the following month they took £120 extra. I contacted them and found out this was due to catching up with the arrears.

I advised them about the time i was off sick and they informed me that they had never received proof of my sick pay and that they had sent a letter stating that the matter was now closed and could not be looked into. but, i never received this letter. i have argued with them now on a few occasions regarding this matter but they will not look into the matter, even though i have provided the proof again. how can it be ok for me to make a £400 overpayment once my arrears are all paid?

Is there a way of appealing this as they say this is not possible, but how can that be, if they were to consider an average of my earnings over the course of a year this would affect the amount i pay in regular CSA payments.