Backdating Private Maintenance Agreement

Hi there, am very new to this so apologies if my questions have been covered previously.

My partner has a 5 year old child with his ex wife who he has not had any contact with since July 2014 as his ex blocked access when he left the relationship as a result of her domestic violence towards him.

From the month he left he has been paying her £300 a month via standing order as a private agreement, neither have gone via CMS/CSA. For the first 2 years the CMS calculation states he actually over paid by £50 a month.

In 2017/18 tax year he did a lot of overtime and earned more than in the previous 3 years but did not alter his monthly payments. His pay has now decreased by £5k per year and the CMS calculator states he should only pay £290.

Even with 2017/18 being a higher earning year it still works out that he has paid her more over the last 4 years than the CMS/CSA would have claimed.

His ex has been in contact for the first time in 4 years claiming that he should be paying £330 per month. We're not sure where she's got this figure from as it still doesn't work out with what he earned last year.

So my 2 questions are:
1. Can she have found out what he earned last year?
2. If she chose to go through CMS now would they backdate based on 2017/18 higher salary or on the current lower salary? I have read that a private agreement is just that - private and the CMS don't get involved in backdating/over payments etc

Thanks in advance