Dp and his ex have always calculated maintenance as per CSA calculations and each year, dp would send exw his payslip to prove his income.
When dsd2 moved in with us, she refused to do the same but started paying £5 a week. We pay for school uniforms, school trips, driving lessons, all clothes for dsd1 and 2 and clothes at ours for dsd4 etc etc. Ex w contributes nothing at all towards dsd2. We even provide money for her to feed herself at mums.
The day the new child maintenance system is brought out, ex wife puts in a claim and claims that dsd1 never stays at ours despite her living with us a third of her life for the last 8 years.
The child maintenance people have stated that they will not reduce maintenance for overnights for dsd1 as ex wife won't admit that she stays with us. Our court order only applies to dsd4 as the other 2 were deemed "too young" to be put on a court order despite being under 16 at the time. Dsd1 was over 16.
So the moral of the story is.. If you can't afford a court order (which many out there can't) or the kids aren't put on one because of their age, then chances are you will pay more maintenance if the ex is obstructive.