Don't take ANYTHING these barstewards tell you at face value. One of my nephews was in a very similar position and it nearly drove him to suicide. It turned out that yet again the CSA were in the wrong.
Query it and demand proof and DON'T pay a penny until then. if you start paying they will take it as an admission from you that you do owe the money.
Also, ask them what they plan to do with the money. I bet it won't be going to your ex.
I find it bizarre that they are doing this nearly a year after your case is closed. Another suggestion is to contact your MP. All MP's have a direct line to the CSA and can get results quicker than you.
Good luck.