Question about Annual Review and backpay

Hi All

I'm a first time poster here, long time lurker, in which time I've learned a lot, so thanks in advance for any help or knowledge you can send my way

I've been divorced 6 years, and for the first 5 years had a verbal agreement re: maintenance which was always paid promptly and never missed. However myself and my now partner had a child in May 2023 and in October 2023 we got remarried. This prompted my ex, who I've always had an "ok" relationship with to effectively nullify the verbal agreement, and raise a case with CMS in December 2023. I went through the process and declared everything, including accurate salary etc, and they used (I assume, possibly wrongly) my 2022-23 P60 to tell me what I needed to pay - it ended up being about £290 more than the £600 I was paying. I accepted that and have been paying it since.

More recently, I got a bonus as well as a payrise at work (March 2024) - I did not know I would get a bonus at all when the initial case was raised and decided. The payrise is negligible (it's about 2.5% more, one of those annual things they do to appease employees). The bonus was quite decent, based on good company performance and personal performance. 

At the time I just assumed that will show in my next P60, which it did when I got it in May 2024. I spoke to CMS anyway who said that there was no reason for me to declare my 2.5% payrise, but I should declare the bonus. In the end I went online and declared both, which updated my earnings to about 24% more than they had on record, so they said no upward adjustment was necessary (but this new amount will be taken into account when the annual review takes place (December 2024))

The issue I have is that the information I have provided to them is accurate, i.e. my current documented gross salary, pension contributions, bonus I received in the last 12 months, and that doesn't push it over the 25% threshold based on the information they got from HMRC in December 2023, so doesn't require an instant adjustement

However! The P60 I have for 2023-24 has a value that IS over the 25% threshold. This is because in October 2023 I was awarded 11 months backpay for a delayed payrise that occured before the case was raised with CMS.

I know that the new figure I have given them will be taken into account on the annual review, and since I have been as truthful as possible with CMS regarding my earnings, both at the time of going through the initial case, and updating my salary to give my bonus etc, I have nothing to fear in the sense I've been hiding earnings, but I'm sure when the annual review comes they will only look at the P60 value for April 2023 - April 2024 (which includes backpay I got and the bonus I have declared).

Will they look at that and just make an adjustment based on that value from the date of the review, or will they look at it and say you were earning more than you said you were when reviewed the initial case, therefore you owe back pay.

Totally confused, and chatting with CMS online or on the phone hasn't helped, their only advise was to update my earnings in my account on the website.