Hello, love! This is all so horrifyingly familiar. I was also told I would get a nominal figure (subject to him reviewing my spending) which was very insufficient for my outgoings. I had no option but to take him to court. But it's not a case of 'getting him' on anything you can - divorce is all about the money, and nothing to do with justice.
I'll try to explain SM from what I've experienced.
Let's say he earns 4K (net) per month.
Let's say he has outgoings per annum (e.g. including insurances, pension contributions, monthly bills, normal way of life, rent mortgage etc) of 2.8K per month.
That gives him an excess of 1.2K per month.
He's offering you 850 per month (so a difference of 350pm).
To take him to court it will cost you around 3K (solicitor-led) so it will take you 8.5 months to break even (and in the interim you still have the bills to pay).
That's why SM claims can be a Pyrrhic victory. I cannot stress to you how lucky I was to get my court costs covered (it truly is a rare thing).
It costs a ton of money to enforce your rights/his responsibilities. It absolutely has to be worth it.
Try as much as you can to persuade him to pay the mortgage/bills as well as the children's maintenance.
You know his earnings (more or less) so work out the maths as to whether it's worth it. The best thing you can do right now is file for divorce - as this starts the process of negotiation (and yes, your earning capacity is taken into account - so that will factor into account include your illness). As soon as you get to the negotiating table, then you can discuss with him the children's and your needs. If he can't accept them, then you have recourse to the law.
Talk to your solicitor about a "timetabled" divorce which forces him to comply with the divorce procedure (allegedly) and forces him to the negotiation table (again this takes some time).
However, I stress to you the legal system is costly. I am in a ton of debt now for legal services because SM doesn't take into account the fact that you have legal fees to pay. I got 96% of my claim, which was all functional outgoings (no frippery) and I'm still in a ton of debt because of the legal fees. In every case up and down the country, the dependent spouse faces the same cold hard fact. When he fucks off, he doesn't just break your heart, he breaks the bank account too.
Horrifying isn't it? The only 'good' news is that when you get to settlement, you get your entitlements - that can take quite a while in the absence of a timetabled divorce.
However, do go on the CSM calculator to see if 850 is correct for the children.
Hope you're doing okay - I really feel very sorry for anyone going through this. It is truly terrifying and awful.