Ex demanding I repay child maintenance

Hi, very new here.
A little background- husband moved out in December 2019. He has resisted the idea of divorce, saying he had made a mistake and wanted to try again. I didn't want to
​bit then there was family stuff so I backed off for a time. Now I want to get things sorted this year. Ex has a habit of turning nasty when he doesn't get his own way. We haven't really spoken since last September when he went on a rant over the phone about how my brother is a c#nt. (Sorry for the language and the long post).

Anyway, he has paid child maintenance since he moved out. There are no legal agreements, he worked out via the gov website and paid the amount it said. In 2022, he said he had overpaid due to being furloughed during lockdown.
I repaid what he said was overpaid.
Now he says he has overpaid again as he was made redundant last year. He has continued to pay until January of this year and then suddenly stopped. When I queried it I have received an email saying 'Congratulations. This is what you get when you issue an edict via ****(child name) that I will not speak to you unless... and is demanding repayment. My question is, can he legally demand repayment given that he knew his circumstances had changed and he continued to pay regardless?

Sorry for the long post.
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No he can’t.

If it’s a private agreement you can pay any amount you like and can stop at any time.

He can you for it back but has no legal entitlement to it.