If I move house I don't really need his child maintenance ( he offers lower than what he gives to his eldest whereas we have two kids!). I can't even be angry with that!
Child maintenance is money he owes to your KIDS. If you don't need it for day to day expenses, put it away in savings for them later. Odds are he won't be able to help them with college expenses when the time comes, you can put it away in a college fund for them. There are formulas for figuring out the child support amounts.
