rickman, I sympathise, I really do. My ex-dh purposely gave up his job so he didn't have to pay me any money. And for some reason the CSA stopped pursuing him. Although it couldn't be proved, he was working cash-in-hand for a couple of mates. When I snitched on him, he told them he wasn't taking any money for the work; that he was doing it for free to help lift his depression and give him a sense of purpose! wtf?!
If your ex is paying less, wouldn't that mean you would be given more from child tax credits or income support instead?
As tempting as it is to prevent him from seeign the kids (I was fuming so much I was sorely tempted too) you must be clever when dealing with him. Easier said than done, but don't let him see you mad or upset. Don't get mad, get even. It may take a while but he will get his come-uppance.