Off topic advice on child support agency

Self employed/limited company, pay yourself the bare minimum (I pay myself £208 a week). The CSA guidelines on the amount of maintenance, on those earnings is about £22 week. Dont know what their view is on expenses/dividends, or even if they’d need to be told, but if you go the self employed route, either as sole trader or limited company, get yourself a good accountant.
You obviously dont begrudge paying for your son, but your ex sounds like she is out for what she can get, and milking it, and a good accountant could put your figures in a more favourable way (for you) to minimise how much the greedy cow can take.