I had an ex who wanted to extort me for £600 per month while also expecting me to pay for my own costs like travelling to see my daughter, now they lived hundreds of miles away.
I ended up using the government calculator which suggested something like £60 per week, which was much smaller than the £600.
I referenced this and mentioned that I would give extra money "as required" for things like clothes if she sent me a picture of the receipts.
She vehemently refused this simple request, and was incredibly annoyed by the recommendations made by the government to pay £60.
Since I was not refusing to give more as and when required, she felt it was my obligation to also pay for nice things like swimming lessons, even though the swimming pools are free for her to take our daughter.
When I moved to be closer to them, and took my daughter half the time. I stopped paying at all since I had shared custody and was responsible for my own costs. I said that any extra stuff like school uniforms she can request the money from me. Since the remainder of the child care at that point was so low it was essentially for clothes only, and stopped being about food and shelter, since I have my own costs related to feeding and sheltering my child when they're with me, so I stopped it entirely and this caused a lot of outrage but my argument was sound. 50/50 split, and not refusing to pay for things that were shared costs like school uniforms. I buy clothes for other times for my house, and she for hers.
She angrily opposed but she knew that I was right. If I have my daughter for half the time, and she has her for half the time, why is anyone paying anyone? Similarly for your case, they are responsible for 1/5th of the costs your kid is with them.
The maintenance costs cover things like clothes, shelter and food, not for things like child care or after school stuff or extra-curricular stuff.
There are, in fact, other ways to provide the clothing aspect too, so if he buys clothes or says "send the receipts and I'll send you money" then technically he is fulfilling part of his obligations and so the remainder of the child maintenance is for food, so it could be reduced by the clothes contributions.
Honestly I have no idea why some people believe that they are entitled to more than this like my ex thought she was entitled to £600 which was over 3x as much as the government recommendations. The government recommendations are pretty decent without making life hard or unfair for the NRP.