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Your employer should have a "Right to Apply for Flexible Working" which you can use if you are a parent (other groups also have this right). Ours has a form attached to complete, but a letter is usually acceptable.
You need to state what hours you require (think about any flexibility you can offer with this), why you require this working pattern. Think about how it may affect your employer and if you could minimise any impact. I'm going back 2.5 days but my DD is in nursery 3 days, so I could offer to swap my half day if they needed me to.
Send this as soon as possible, so they have time to consider it.
If they refuse you, they have to have sound business grounds for the refusal.