CSA and repayments/ineligibility

Munchkinsmum

After a year during which my son was primarily living with me, but then changed to primarily living with the other parent, I was contacted by child benefit who said they’d decided the college course he was on wasn’t eligible for CB so I needed to backpay the amount to them. I informed CSA (as I’d claimed CSA for part of it and was paying the rest) and they closed the case as clearly the other parent had received the same letter.

My query is, is nothing done about the months of payments where the parent wasn’t eligible to receive it? I’ve contacted CSA who say they’ve closed the case and aren’t interested. What irks is the other parent opened a tribunal regarding the starting four months of this period and them having to pay, which is presumably going to court and I’m expecting now to have to pay it back, not for the reasons they opened it but because strictly speaking I am now considered not eligible to have received it as of back then. But I’ve paid so much since then that it would technically cancel out - but apparently that is just a loss?