NC. Not sure if the best place to post but I know this is a knowledgable and supportive board 
My stbexh pays CM of £400 a month for our 2 children. This used to be via the CSA but they no longer handle the case since it went over to Child Maintenance options. I'm not sure if this is the "correct amount" he should pay as not sure of his income (he does a lot of overtime). We are in the process of sorting a financial agreement for the divorce so I'll get to know then. Until then I don't want to rock the boat as he says he's paying me more than he has to. It gets confusing as he has 2 more children with his new gf. She has several existing children. He claims to live with a family member but he lives with her. There is literally no room at this family members house for him and he has no belonging there, his car charging point is not there etc. He pays his gf maintenance for their 2 children but only pays her half what he pays me (I know this from his bank statements previously shared for the financial order 2 years ago. This could have changed since of course). I'm not sure why he pays her as his wages go into that household 
Things are very strained between us as he is abusive and communication I currently via my solicitor.
Our eldest starts secondary school next month and he's not offered either any extra money or to buy her anything himself. I did ask about a year ago if he was going to contribute but he said we'd deal with it nearer the time. I originally wanted things like this included in the financial order but he's been delaying this for the past 3 years so we are still arranging finances.
Would you ask him for contribution towards uniform and things she needs? Or not rock the boat? He is financially abusive and sometimes threatens to cut the CM if he doesn't think I'm spending it appropriately. We have a very modest lifestyle. Modest to the point of having about £20 a week to my name to feed us all at times when I've had e tea expenses (like uniform!)
On one hand I don't want to ask him for help but am struggling 