I overpaid CSA now they’re making up debts

I have two sons with two ex-wifes.

I first contacted the Csa in May with regards to the eldest son turning 20 years old so the payments could be adjusted.I was told at this point that his mother had contacted them on the 27th sept 2013 to say he had moved out. At the time i paid a total of £168.30 each month.

They had not re-calculated the fiquire until i phoned them. My new amount for my youngest son is now £109.00 a month.

So for 21 months i have been paying approx 59 pounds to much.

After the adjustment for the 2nd child she got £866 pounds paid to her. I was expecting £1190.00. I recieved £314 I asked them for a detailed breakdown on how they came to this fiquire.Today i have received it. In the second section there is a part which says Uneforcable Debt total Charges =£876.which makes it all balance.

However i am not in any debt with them and have always paid when i have been in work.

Due to ther office maladministraion back in September 2013 i have overpaid £1190.26 but only received £314.00 back.

Any help or guidence on this would be greatfully accepted.