The purpose of the CSA is basically not to support the mother or the father or even the child. The purpose is to help the taxpayer.
So if no-one is on benefits, the CSA will be 100% happy for you to come to whatever private arrangements you want.
But if one or both is on benefits and the other one can pay cash to support the people RATHER then the benefits be paid -- obviously the CSA is going to try and make that happen on behalf of the taxpayer. Everyone slates the CSA because they don't realise they are there for the taxpayers benefit. So when you say things like 'Why can't we just have a private arrangement' the reason is because one or both of you is still probably going to be ***** enough to be claiming off the state AS WELL if you do this. If the CSA enforces something -- THEY'LL tell the benefits office -- the benefits will be cut accordingly and TAX PAYERS LIKE ME benefit by having lower council tax. Or so the theory goes ..
So you have blokey earning £100,000, with a private arrangement to pay ex-wife £1000 per month -- and wife is ALSO half-inching full benefits off the taxpayer because she doesn't bother declaring the private arrangement. THAT is why the CSA exists. If neither is claiming benefits -- the CSA won't give a flying moneys about you ..