Contact the child maintenance services. They’re government run, and it won’t cost you anything. Give them as much information as you can about your ex so they can find them – they’ll calculate how much is owed based on your ex’s earnings.
You then have the option to have it put in your account by your ex, or get the child maintenance service to collect it and pay it to your on your behalf (probably a better option given your ex’s reluctance to pay it) it will cost a bit extra to do that (they add an additional 20% to what your ex pays and take 4% off what you’re owed) but then you don’t have the hassle of chasing your ex all the time.
It can take some time for them to get it all sorted, but once you’re in the system they’ll review it every year to ensure you’re getting the right amount.
Him having another child will affect the amount your child(ren) can receive, and if he has his own business he may be able to make it look like he doesn’t earn much (and may already be doing so to pay less tax) but something is better than nothing if he’s refusing to pay.
