They either make things up as they go along or it’s just a complex service

Any advice would be greatly appreciated. Whether its because the CSA make things up as they go along or just because its a complex service I’m confused.

My fiancé and I have been together for 4 years and have 2 young children of our own, 18 months and 6 months.

My fiancé was married once before and has 2 children from that marriage. They are 19 and 14.

This is a very sad situation but my parter has not seen the his eldest two for 10 years. At the time of their divorce it was a very horrible situation to be in, my partner had met someone else and as a result the Ex Wife made It all but impossible for my partner to see the children. From what I can gather the whole situation was unpleasant and after things that were said to the children by their mother they decided they didn’t want to see their Dad anymore either. We have recently discovered that they have both changed their surname by deed poll and the youngest has no memory of her Dad.
My partners Ex Wife has openly said she wants him to have nothing to do with them also despite being loosely in contact with other of his family members – of which makes no sense at all to me.

My partner has paid maintenance since they separated all those years ago, originally it was a case of my parter having the kids every weekend and giving her money every month towards the upkeep of the children, even when the contact with the children stopped he still paid maintenance but once she realised she may get more money by going through the CSA that’s what she did.
Years went by and he duly paid the monthly about they owed when he received a letter saying after a review of his case the anything he owed was now zero. (All of his details were up to date which reflected in the paperwork from the CSA so we assumed the Ex wife had cancelled the claim)

My partner received a letter today from the CSA informing him they have reviewed his assessment (unsure why as there’s not been a change in circumstances) and not only has his payments increased but they’ve now attached a DEO for £603 to come straight from his wages every month. He brings home £1475 Calendar monthly. This seems obscene.
He called the CSA today and also informed them that as his son had turned 19 at the start of this month and was working full time the payments should now cease for him. Apparently this is something they have to look into before they can reassess anything but regardless they still gave no indication as to why the payments have increased, or why a DEO has been issued when this is usually used as a last resort for non compliant parents?

I really hate the CSA. I fully agree that it takes 2 to have a child and the absent parent should provide for their flesh and blood but shouldn’t it also be fair all round?
The ex wife remarried. They own their own home, have 2 expensive cars on their driveway, luxury holidays abroad and both in 60k+ jobs.
My partner and I live in rented accommodation, both have menial jobs (I am on maternity leave at the moment also) with 2 very small children and can barely afford to live as it is without the CSA demanding more money. Our children are missing out so much already – before anyone slates me for having children we cannot afford, we were financially in a better position than we are now when we decided to start a family. We face the very real reality of being made homeless.

The CSA claim they aim to lift children out of poverty. My partners children aren’t in poverty, one of them is deemed to be an adult now (and doesn’t live at home with his mother!) and to pay a ludicrous amount each month for children who 1) probably don’t see the money and 2) wouldn’t go without if they didn’t have it, compared to 2 babies who depend on that money is just absurd. My children are being robbed of the things they need to fund their siblings who aren’t going without? Over £300 for each child and yet we cant even afford that on our 2 each month. How does that make sense?
The CSA have been anything but helpful when my partner has contacted them. He’s not said be doesn’t want to pay, he accepts his responsibility and wants to do what he can and should but all he’s asked is for a reassessment and a fighting chance to provide for his family unit and young children that live with him. Unsurprisingly the CSA aren’t interested and just speak to my partner like dirt when he tries to call them. I really loathe the CSA and the mothers who’ll do anything to spite the ex because they can.
It seems as though the CSA are in breech of certain Human Rights but are a law unto themselves. I really have no idea what to do. I’m frightened for our future and all I want to do is cry for my babies, I know its unrealistic to expect to be able to give them the world but barely being able to provide them with the things they need pains me beyond belief.
Can anyone shed some light on what we can do?