Clubs, activities, school trips and wrap-around school care, who pays when?

Hello everyone, first post of probably many, but certainly the most urgent.  My understanding is that regular clubs and activities should be covered by maintenance payments, my children's mother's understanding is that I should be paying for half of those on top of maintenance.  I have been paying for their music lessons, purely because it won't get done otherwise, is there any way to get her to take responsibility for these?  But my biggest burden is coming from the before and after school care, I see the argument that they meet my need and therefore I should pay for them on days where I have care, but I also wonder whether that is technically accurate since if I didn't have care of the children on those days, she would still need to put them in the clubs (she does everyday and frequently for longer hours), and they are regular clubs.  Does anybody know with certainty whether I should be paying twice?  On top of this, she insists that I am responsible for half of school uniform (which I provide separately anyway since they go from my home 3 times a fortnight and I am fortunate enough to have plenty of donors of the generic stuff) and half of school trips,; the literature I can find counters this claim, which is correct?  I do not begrudge my children anything, they enjoy the same standard of living in my home as they do their mothers, but the financial burden is crippling me and I earn fairly well, but have nothing left to do anything with, whilst she enjoys luxury holidays, in my eyes at our children's expense, since in 6 years she has not taken them anywhere.  

To give you a fuller picture, but not to deviate from the original query of childcare costs yet.  Their mother earns substantially more than myself now, and works much longer hours for it, is the pwc and child benefits claimant despite earning above the threshold whilst I remain just below it. and I hope to challenge her for 50/50, but she evades mediation.