CSA refuse to sort out their mistakes

Before the csa were invented i payed £30 a week to the mother of my child, i earned £150 a week at the time, then they got in touch and said to keep paying the £30, a few months later they came back and said i had to pay £52 ! not only that they back dated it to when they got in touch so i had to pay £10 on top to clear the arrears ! then in july 1999 i owed £163.63 in arrears, which then jumped to £12,340.03 september 1999 !!! i have pointed out this is insane and it must be a computer mistake or something but they point blank refuse to sort it out and now i am paying them thousands every year !!!