CSA reassessed me, costing me a fortune in back pay

Here goes, way back in 2008 the CSA took nearly 8000 quid from me when I sold my house and told me this was arrears from me not paying, this is despite the fact that they were constantly over assessing me and I was waiting for the correct assessment. Anyway time rolled on and I got a new family, a mortage etc and was reassessed again.

It came up that I didnt earn enough to pay the CSA and they backdated my case to the day after my house sale. So I obviously called them and asked why they hadnt assessed me properly all the way back to the beginning of my case.

The answer I got was laughable. They told me they couldnt backdate reassessments, despite doing it for me and that the 8k was theres and tough.

I got a breakdwon of the assessment that created the 8k and they reckoned I earned 30k a year, take home, when i took home 12k.

Is there anything I can do to sort this as I think the CSA owe me a lot of money!

Cheers