Advice for single father

Hi i split with my ex around five years ago we have two children and my son moved in with me so we each had full time care of a child each.
A little later i found out my ex had approached the csa demanding money (strange i know) so i called the csa an explained about me having my son full time etc their repy was i would need to open my own case aswell (nuts i know)
Long story short my ex had become self employed and used it to claim she earned £100 per week full time.
So to this day iv recieved nothing had a liabilty order taken out nothing came of it csa have now written saying she owes £3600 in arrears but i have to continue paying her the £140 a month child maintance i am responsible for surely i can stop paying until her arrears are paid off?
Please help