Why do I need to pay arrears if they knew about my standing order?

Hi, for quite a few years I was paying maintenance to the csa but 2 years ago I got a new job.

I contacted my ex and we agreed a private arrangement which I would pay her direct by standing order, I then called csa & they confirmed this was fine.

Now 2 years later the csa have arrested my wages for arears from the last 2 years??

What can I do?

I was lead to believe I had an agreement privately, I am not disputing what they want me to pay but I am disputing the arears as I have been paying through standing order. Help!