He is helping with the children, he is giving you £20-40 a week. This is definitely not enough I agree and he should pay more but you are using this money to pay debts when your children are going without.
You should be able to give them snacks when they are hungry and buy clothes appropriate for their age but you can't because you are using the money he gives you for other things.
Can you go to the Child Maintenance Service and get him to pay what he should?
https://www.gov.uk/child-maintenance/overview
And stop feeding him, you can't afford to feed yourself so you definitely shouldn't be feeding him!! Just say 'No sorry, I can't afford to buy food to feed anyone ther than me and the kids'.
//Stupidly I still love him, even though he mentally abused me for years due to his illness. I cant seem to let go. // I think this is part of the problem though sadly, you need to be strong and make a clean break from him. Yes, allow him to see the children but staying all weekend and visiting most evenings is too much, it is not helping you get over him and you are supposed to be separated. He is taking the ***.