How much more does my ex need to pay for his children?

My ex husband has cut the money he gives me for our 3 children, he only pays for 2 at 31 pounds a week, He has now informed me that he no longer wants our 10 year old daughter, as it upsets his weekend, he only wants to see the middle child.

I would like to know how much money i am intittled to, he is a long distant lorry driver on over 32 thousand pounds. He now lives with his new partner, althou they are claiming benefits (housing, Poll Tax) his post is deleivered to his parents house and she works 8 hours a week, i would like to know how much more he needs to pay for his children.