How do you word a flexible working request?

[babybouncer]

I'm two weeks from the start of my maternity leave and I've approached my employer about the possibility of dropping to 4 days a week (as a teacher I thought it important to ask before the new timetable is done) and they've asked me to put it in writing so they can consider it. They are not especially pro or anti the idea and have said they genuinely need to look at it to see if they can accommodate it. I also have the issue that I have a TLR (paid responsibility point) for being head of subject, which they are suggesting I may have to drop if I'm not in everyday, although as I'm only dropping one day I may be able to keep it.

My question is how do I word the letter - what do I include and how do I also ask about keeping my TLR?

Also, when do they have to let me know their decision (obviously it's a long way in the future, but I will need to know for childcare)?

I'm on my phone so can't link but search for flexible working request letter template. It's on the government site but you basically say what you're asking for any why you're asking for it, it should be sorted a while before you go back but being a teacher I suppose as you say it benefits them to know in advance. It could be sorted quicker.

Thank you!