Thanks everyone. I saw the Solicitor today and started the 'legal separation' ball rolling and got lots of advice.
He will only have to give me 20%..... unless we come to an agreement. I spoke to him today before the Solicitor as he got paid into my account. He was sitting in a car, with no petrol and no money to buy food, waiting for me to transfer money. I transferred all but £450 to cover half the childcare and the car (which he has). There was a problem at the bank with their systems and the money I transferred at 9.30am didn't reach his account until 3pm! Can you IMAGINE how hungry he was by then?
I did feel sorry for him actually. He's like a child.
Anyway, basically I'm going to tell him that I need £450 per month. He'll go along with that unless his Mother gets her claws in.
I'm not entitled to any tax credits due to my earnings, in fact I just found out we OWE them £4000!!! Just what I need. 
Solicitor advised me to draaaaaaag that one out as there have been so many cock-ups with the Tax Credits they'll probably not chase too much.
Anyway, he went to my Mums to see the kids today whilst I was working and spent 10mins crying, couldn't even speak to them. I think the hardest thing was my dd, who is SUCH a Daddy's girl, wouldn't go near him. 
He's back at his folks now. He's going to phone every night to say Good Night to the kids. I think he got a real fright when he saw how quickly he had become a stranger.
I'm doing okay at the moment. Just worried about my job. I'm a Contractor so could lose my job at any minute!!! At least H's income was solid. I'm sure I'll manage.
It really helps to have MN back. Thanks everyone. 