CSA making mistakes with my arrears

I have been paying £52 arrears a week for the last 14 months to the csa which is taken straight from my wages. I also pay my ex nearly £80 a week in maintenence by direct debit. I have just found out that the csa have been sending the 52 pound arrears to my ex every week by mistake. This amounts to £2,500 pound which should have come off my arrears.

The csa have now told me that there is nothing they can do about it . Surely this is not right? Please help as I am at the end of my tether.