You absolutely must take the time to draw up a list of the family's monthly expenses by yourself without any interference from him.
Rent
Food + school lunches money if your kids don't have packed lunches
Toiletries/Cleaning materials
Gas
Electric
Council Tax
Water Rates
Childcare
Childrens clothing
Childrens haircuts
Childrens weekly pocket money/school bus fares if relevant
Do a spreadsheet or handwrite it. Tot it all up and split it 50/50.
Present him with his copy and ask him why he's not prepared to pay his 50% ? Why has he been knowingly underpaying for years and letting you pay much more than your fair share ? I'll tell you why, because HE is "money grabbing". it's just that he's doing it by stealth by not contributing his share. Do you think he's been putting some of his earnings into savings in just his name or pension ? Whatever his excuse is, he's financially abusing you and the children. Think about that. Abusing you and the children, albeit financially.
Point out that you'd be highly likely to get more than £200/month in child maintenance if you split up and he'd have to pay rent/utilities/food on top of that if you're no longer living together. Meanwhile, your household food bills would reduce and you'd be able to claim single person discount on the council tax bill for the place that you rent. So you and the children would potentially be much better off financially.
Be prepared for a surly response though. He's got used to a certain level of personal spending money and won't want to have to economise.
I couldn't bring myself to have 1 child, let alone 4 with such a financially abusive man.