https://www.acas.org.uk/making-a-flexible-working-request/if-you-want-to-appeal-the-decision Acas is normally good at giving info re what to write on these things if you've not looked at it already
Speake with HR I had big issues with childcare when my son was poorly in hospital when he came out I didn't want him away from home so I had to ring in sick for one shift and they tried to change my shifts from nights to days but ibhave dine nights for 7 year and can't do days for childcare. I spoke with HR, and they said the aim is to not make it harder for me and to work with me. It's not your problem they have to sort someone to do a Sunday you can't leave your child simple as that x
On speaking to HR all they say is email it across and the case handler will speak to your manager. I just feel so deflated at the moment such a difficult situation.
Care work is always needed, if it were me I’d say to them that they either let you be flexible or lose you completely. Obviously I know that’s not sustainable for everyone but could be worth a try