Why must I pay back the money I was owed?

Hi there I am looking for advice. In 2009 the visa became involved in my case. My kids dad had started working 8 months before + had been paying me £20 a week. The csa done a calculation + decided as he was earning a good amount he was to pay £80 every week.

He decided to pay me direct however as he hadn’t been paying the correct amount they would collect the arrears of over £2000 + pay them to me. I received these payments until the arrears were paid off.

Yesterday I received a letter telling me I’d been overpaid. Guess how much, yes it’s the arrears they have collected! I phoned them but it was like banging my head against a wall. They said that yes he was due me this money but as he pays me direct I should have arranged to collect the arrears from him.

I explained they had sent me a letter stating they were going to collect arrears. I was told that I shouldn’t have received this letter! They now want me to pay this back to them, then they will give it back to him + I have to arrange to get my arrears from him. I hope someone can help. I’m worried sick about this. Luckily I have kept all correspondence from them. Any advice would be appreciated. Thanks.