How do I know what the CSA is telling me is true?

Hi i am hoping you can help,

I have had two cases run by the csa one of which has just come to an end,i found out in late march that i had in fact been paying the csa for my eldest son for eighteen months too long.

They have been in touch with me to say that i have over paid nearly £1500 but instead of paying me my overpayment back they said that it will be used to pay off arrears on my other case.

I asked them how they have got a figure of £1200 arrears and they told me that they didnt know as the computer system was not giving them the right information.also i pay by detachment of earnings and find it hard to accept their claims.

Please can you advise me if one they can offset one payment with another and two how do i find out if the arrears they are telling me are correct thankyou.