I thought that taking money that didn’t belong to you was theft?

Letter from CSA states they took too much money, direct from my wages, and that I have overpaid 350 pounds.

I no longer earn anything and don’t have to pay but in a letter to me last week they state they have decided to keep my money and apparently I cannot appeal against it.

In this country taking something that doesn’t belong to you keeping it is theft… How can CSA be above the law and how can I sort this out??

Any help would be appreciated.