Getting money from the CSA

Quick version is the CSA put a charge for £5700 on my ex partners property as he was saying he would sell it and now he lives in spain.

They got the order, went for the second order late and the flat was repossessed two days before, so I never got the order. Now sold it, took months and finally they received some money (no communication with me) and without me knowing paid into my account the amount I am owed fro my daughter, in total over £13,000 and they have not written or called, just paid it in and now said it is an error and can I return the difference in £5700 order and money paid in but admit I am owed £13,000.

Now they will let my ex who will not pay have the difference, this has been a nightmare of at least 8 years and calls letters and no help? I am owed this money can I say your error it is what you owe me? please help.