Asking for set days. How do I go about it?

[mmmmmchocolate]

Hi all I'm hoping someone can point me in the right direction.
I'm due back at work soon after having DC2. I previously worked 3 days, contracted for 20 hours. Mil has DD1 while I'm at work but wouldn't cope with both DC. So DD2 has to go to nursery/childminder. This means I can't work Saturdays as no nursery in my area is open Saturdays. I also want to work set days as I can't chop and change what days DD can attend nursery. As it's costing me a fortune for childcare I also want to drop to 2 days.
So what do I do? Do I just outline in a letter/ email what I want? Tia

[BarkisIsWilling]

Firstly, does your org. have a flexible working policy, and have you checked it?

Next, have you gone on the direct gov uk website and searched, yet?

Finally, I'd suggest trying a search on this site to see what others in similar positions to yours have been advised.

All the best.

[mmmmmchocolate]

Thank you barkis I've just found the gov site and there is an example of how to write a request letter. Many thanks for your suggestion :)