AIBU to apply for CMS after years on £200 monthly?

[StitchHappens]

Really quick AIBU.
My ex pays £200/month for 2 kids (17 with autism/special needs & 14). This amount was agreed around 6 years ago when he was employed and living alone. He has since gone self employed and moved in with his partner, so I was wary of applying for maintenance via cms as I have read a lot about self employed people doing cash in hand work and therefore the maintenance payments potential to be lowered.
He has now said that they are buying another house (moving not an additional one), and I am presuming he has applied to be on the mortgage and would therefore be declaring all his income.
Aibu to apply for cms without telling him.
I have asked several times in the past if he can up maintenance and been met with a flat out no.
I have just had to drop to 12.5 hours at work in order to meet my elderly child's needs since they left school and started college (course is only 3 days per week).
Child 1 never stays at exs.
Child 2 stays 1 night per week and some holidays (3/4 weeks per year max).

Sorry, that wasn't so quick after all!

The only thing that matters is his income. Houses, moving in with someone..unless they have kids he can include to discount maintenance, is irrelevant. SE is notoriously tricky as its very easy to hide income.

YANBU. He can try to hide his money if he is self employed but when I worked at CMS years and years ago (so it might have changed) we could ask for a copy of his mortgage application and see what I come he had declared and use it for a calculation.

Yeah go for it, you owe him nothing.

He hasn’t even accounted for a yearly increase based on inflation? I’d contact him and say you haven’t revised the amount for 6 years- cost of living has gone up can he increase the payments to £300 a month. When he says no go to CMS, but when he kicks off at least you can say you contacted him, set out a reasonable increase and he declined.

I understand that, but if they are applying for a mortgage then he will be less likely to not be declaring income.
I do know it's a risk as I can't guarantee either way, I just can't see how the payments would go below what they are now.

No. I asked if he could increase payments less than a year ago, and was told no.

Have you got a rough idea to the nearest 10k of what he might be earning? My concern would be if he’s on 25k a year you would be better off whilst receiving for 2 children- but you don’t have that long left for your eldest.

I'm not entirely sure, as i can only go on what he tells me he's working, and I don't know if he uses work as an excuse not to have them more.
As a carer the council page says he charges £20/hr. He says he works all day mon, wed, fri, Thurs & sat till lunchtime and then in the evening, Tues & Sunday evening (I think the evenings are only an hour or 2, putting someone to bed).
They also have a small shop they run between them, but I discounted this as I don't know who's name it is in.

Eldest will be in education until he is at least 20, ehcp goes up til 25, and they are unlikely to be able to get paid employment.

Thanks to all who commented, I bit the bullet and applied via cms and despite him being self employed the monthly payments have gone up by around 40%, which is significant to me. This is based on the 2025 tax year, so I would expect them to increase again next year, but more importantly to me, I now don't have to go begging him to look at the amount, it will be reviewed each year.

Congrats! Hopefully, he doesn’t pull any sneaky shit!