What do you think is going to change?
Do you think CMS and DWP are going to stop and say, "Oh, actually, we didn't realise you filled in more forms than him, here's both CBs back and you can keep the maintenance"?
They have decided. You are 50/50. You are going to have to pay the money back. What are you going to gain by taking this through official channels? It's been through the official channels. Court isn't going to save you.
I'm SO glad I don't know you in real life, you are absolutely exhausting. I dread to think what your ex has put up with before this latest stunt. Honestly, he needs either a sainthood or his head read for ever having kids with in the first place.