I have over paid the CSA

I was in arrears with the csa for £3000 which i was paying in 2009.At the start of this year the csa say i am still due £3000 to which i dissagreed on so i asked them to send all payments received in black and white to which they did and my god is it really idiots which work there???

It says i paid £11000 in 5 years after looking through all payments i paid £12500 and i was unemployed for 3 month to which they say they never received any letter also one payemnt i made was for £1500 and they say it was only £1170 is someone on the take?

It’s not my mistake as all payments are taken off my wage before i get it so i have the evidence but they will still not back down, they are due me £2800 i have a solicitor involved and it has taken them 5 month to get back to him but still no further forward so now its goin to be court and they will be the ones paying as i have it all in black and white.