CSA wants half my yearly wage

I started paying the CSA by debit card over the phone two years ago and was told that I had to make things more permanent by standing order. I told the CSA that I worked with two nursing agencies and this would not work and they made arrangements to have the money come from my wages at source.

This is not happening and they now say that I owe them 4k.My wage last year from both my p60’s was just over 8k due to my current circumstances and It doesn’t take a genius to see that 4k is not 20%. I have no problem with paying for the upkeep of my children by paying my wife direct but no one will listen. What can I do here???