Apologies if this is long.
Ex and I separated almost 7 years ago and have 3 DC (DTs aged 10 and DS almost 12).
I never thought he was a crap Dad until we split. We agreed on £300 a month CM, £100 per child per month just seemed fair. Shortly after the separation, however, he moved in with his GF and her DC and suddenly halved CM to £150 a month which was a massive shock. He said it was because he now had to support his DP’s children 
. She wasn’t the OW, they’d seriously only known each other for a month at this point. I wish I’d contacted CMS straight away but it was already a difficult period and I couldn’t be bothered with the hassle if truth be told so I just sucked it up.
He’s never been able to have DC overnight because they live in a tiny 2 bedroom house so no space for our DC to sleep. He used to see them for 6-7 hours every weekend, this has reduced further to EOW since covid. 12-14 hours a month, that’s it. Even when they do see him, he doesn’t do anything with them and they just sit in his house playing games, usually in a different room. They’ve never spent time alone with him because his DP and her DC are always there. Very sad situation and I don’t know how he can live with himself tbh but there you go. He barely feeds them when they’re there either so they always come home hungry. His favourite meal seems to be carrots, cabbage and mash (they don’t eat meat and he refuses to buy an alternative so that’s all they get). DS has said it’s like WW2 rationing and he isn’t wrong really…
He’s never bought any extras like shoes, clothes, uniform etc so CM is all I get. He paid £150 pm for around two years then increased it to £270 a month when he got a new job, it stayed at this amount from there on out.
As the DC have got older, costs have inevitably risen. Phone contracts for example, clothes and shoes are far more expensive because they’ve been in adult sized shoes for at least 3 years, they’re growing so obviously eat lots more food and DS’s uniform set me back £300 for secondary school which couldn’t be avoided because almost everything has to be purchased from the uniform shop. It will be £900 this year when DTs start, I can’t pass any of DS’s down because girls and boys have different uniform and PE kit. DS also needs a bus pass and dinner money now which is £100 a month (I can’t reduce this cost at all because I’m not entitled to FSM and the school is 3 miles away so not really walking distance especially in the cold and dark. I can drop him in the morning but I’m working in the afternoon so there’s no one to collect him). This will be £300 each month from September when DTs start. I’ve also shelled out £400 on DTs residential trip, I paid £200 for the same trip last year for DS.
I explained all of this to my ex quite a few times and asked if he could increase CM at all but he said he understood how expensive things were but wasn’t in a position to pay any more. The worst thing was the fact I never knew which date he’d pay CM so each month I’d have to ask for it which I think he enjoyed, like a power trip of sorts. He wouldn’t set a standing order up, I’d asked numerous times over the years so I was always left in a position not knowing when he’d actually bother sending it.
Last month was the final straw. He told me he could only pay half one week then half the next because he’d bought the DC’s Christmas presents so didn’t have enough. The worst thing was probably the fact that when it came to Christmas, he’d only spent £40 max per child. He also dropped them off earlier than usual on Boxing Day saying he wanted to avoid non existent traffic on the motorway and this was the only time he saw them over the Christmas period. It wasn’t the first time he’d done this to me, it happened probably 1- 2 times a year. It left me short just before Christmas when I had bills to pay and I was just fed up so finally contacted CMS.
I thought I was doing the right thing because I now wouldn’t be forced to ask and I’d have a set payment date. I actually feel like I’ve just screwed myself and in turn DC over because they have calculated he only has to pay £220 per month. This is based on his April 2020-April 21 tax return, he was furloughed for 5 months during this period so inevitably earned much less than usual. I did appeal their calculation but they told me he doesn’t earn 25% more now so the calculation still stands. My heart sank when I read the letter and I’m sure he’s over the moon about it. It’s £50 extra in his pocket each month but £50 less in mine and in turn, in the DC’s.
It’s just a kick in the teeth really. If he was a great Dad outside of this and was actually arsed about seeing them then it wouldn’t be as bad but he’s mega crap and now is allowed to be even more crap by paying less for them. I’m honestly so upset but there’s naff all I can do. I work really hard and have a graduate job so a fair bit over the threshold for any extra support like UC, FSM or help with school transport but it’s still a struggle each month. There’s very little else I can do except have a little cry and move forward. It’s so shit. I just wanted to rant and also ask if anyone else has found themselves in a similar situation after contacting CMS, I wish I’d never bothered.