CSA says I owe them money when we have shared custody

I have been told by the csa that I owe them £5013.73 in arrears.

This amount dates back to 2004. I had my 2 kids 3 sometimes 4 nights a week, took them abroad 5 years running. I even had child benefit for the oldest child from 2000 to 2003.

I have paid a total of £22,000 on top of this. I have recieved assessments that vary from £67.20 a week to £86.93 a week. I am self employed. No matter what I say to them they will not listen and state the arrears must be paid, how can this be?

I have done right by my kids.