Well, here is a possible approach:
Dear Mr xxxx, [ the name of the person to whom you are making this request]
At present I am working in xxxxxxx Department as [job title]. For personal reasons I would like to transfer from xxxxxxx Department to yyyyyyy Department. I am prepared to accept a similar salary to that one I currently receive, but would like to be more flexible in working hours.
I hope that this will be possible and am ready to discuss it at any time convenient to yourself.
If you write what you think you should say, we can comment on it.