I don’t mind paying, but not twice!

I have just started a part time job at 16 hours a week and on the national minimum wage.I have recently had a phone call off the CSA to say i was in arrears of over £2,489.58 yet i had been unemployed for 3 years previous to this call yet no letters off them or statements of my £5 they was taken each week out of my jobseekers allowance.So i contacted the Csa and asked them what about the periods when my child was staying with me and my family yet the other parent was claiming for the child and we had nothing towards that childs keep…

They said it was down to me to inform the appropiate departments not them?

So i was angry because this would of meant i was paying to have my child with me staying feeding,clothing etc.and the mother who did not have the child i was in arrears for as she was still being paid as if my child was there living! now i have them on my case over this saying they will take out of my wages via my employer.yet i have had no letter or any notification of any arrers.anyone have any ideas as i dont mind paying for my child ,but not twice!