@ProlongedAffair are you now planning to come to an agreement with him and drop the CMS case?
Your ex, despite everything you've done to get you here, seems like he still wants to resolve this and be amicable.
The ball is fully in your court now.
You can try to salvage something or you can press on ahead and end up in court and very likely a worse financial position when court rule that no CMS is due and that you need to repay what you have been paid.
All of this could have been avoided if you had listened to the hundreds of people telling you the same thing over and over that just because you held a belief that you did more by having them registered at your GP and filling in a few extra forms doesn't make it so.
I'm still really astonished that you sacrificed what was a good arrangement and a good co-parenting relationship for some extra money, did you not think of the impact of your children in doing this?