As another HR bod here, I would also suggest if you are going to put a request in for part time, make a business case such as who will cover the other work, how much (or not) its going to cost them, how it will/wont impact on the job and what you will do to mitigate. It may seem like a pain, but the more you have solutions to all the possible blocks right away, it will leave them less space to wriggle out of it and use the catch all 'business reasons'.