CSA say I owe arrears when I know I don’t

I have 2 boys and the oldest I no longer need to pay maintenance for, however my youngest is 18 soon and apparently in further education which I’m slightly dubious is true. I was paying my ex directly but I was made redundant so advised her of this and said I might have to reduce the monthly amount. Without replying to my message all of a sudden she’s got the CSA chasing me again.

I’ve no problem paying the £28 per week but the £41 per week they want for “arrears” is ridiculous. My ex has contacted them twice previously to say I don’t owe any arrears but for a 3rd time these “arrears” have reared their ugly head. Apparently when she told them, the arrears were only suspended instead of being wiped off.

To top this off my ex took my sons to live 300 miles away a few years ago and I’ve never been a missing dad, I had them every weekend and paid her/the Csa throughout. I didnt leave town she did (with my kids)

After a few months of everything going wrong in my life, being made redundant, splitting with my girlfriend, this is the last thing I need and feel as if the walls are closing in on me. Any advice would be greatly appreciated.