Sounds very poor, but the CSA are a failing organisation.
If your ex is doing clever accounting, there really isn't a great deal you can do IMHO...
The 'going rate' on dad tax is 15% of next income for 1 child, 20% for 2 then 25% for 3 kids. There are allowed deductions for other children he may have - either by a new partner or care for i.e. stepchildren, plus deductions are available if he has additional expenses i.e for the children having overnight staying contact. It starts off at 52 nights per year which he will be eligible for a 1/7 reduction.
Please don't cut the number of o/n's as contact and finance are legally treated differently and it would only be punishing the kids by them seeing less of their dad.
He probably IS pulling a fast one, but how to get around it? Not sure really, sorry.
My ex is a greedy bitch, I have a shared residence order - which legally states that both me and the ex are equal legal parents and both have residency of daughter. I used to pay full-time nursery fees at my employers nursery (£550 per month) voluntarily as the care was fantastic. I can claim NO tax credits or anything - my ex can claim the lot as she has the child benefit book.
When madam finished nursery, I got a snotty letter from her solicitor expecting me to cough up £550 per month to the ex as child maintenance. I told where to stick it as the CSA online calculator came out at £151 per month which I pay by standing order under a private arrangement. My ex - she can claim all the tax credits and I can get NO additional help, despite my daughter living at mine for 35% of the year. The CSA apparently classifies me as a 'non-resident' parent .... what a joke the system is.