Not sure if this is in the right category, apologies if not!
My partner divorced his, now, ex wife a few years ago and since then has paid around £500 a month in child support for the 2 kids. This was agreed outside of court as the breakup of the relationship was amicable and my partner has numerous other outgoings that aren't taken into account when going through CSA (it would be about £750 through them). However on top of this he helps pay for clothes, school trips, holidays, sports clubs among a lot of other things. Due to a big argument between them recently she is now requesting to go through CSA and claim the full amount. I presume she is within her rights to do this even though they previously agreed £500 a month?
It would put a slight strain on our finances and he has said he probably wouldn't be able to afford to help with the other bits he currently pays for. Just wanted to get people's thoughts really, it may look like we are the bad guys but we can only afford what we can afford and we do want to make sure the kids never go without 
TIA x