I agree with what everyone said re checking with your HR dept for the guidelines. I know at my company there's no minimum length of service to qualify for paid maternity leave, but I suspect this is not the norm. Also, I agree with using sick, annual or unpaid leave to cut your hours down as opposed to switching from full-time to part-time work. That way you are still classified as a full-time employee for the purposes for your paid leave.

I'm taking some unpaid leave prior to finishing in order to maximise the duration I can work (due to some things going on at work) but also to manage both my health and work performance. So in my last 4 weeks, I plan to work 4 days a week (for 2 weeks) then 3 days a week (for 1 week) then 2 days a week in my last week. When I suggested this as a flexible compromise, my work were very happy with it. As I am out of annual & sick leave, I'll simply use unpaid leave for those days. Sounds like your boss isn't great to talk to, but might be worth trying to find someone good (maybe in HR?) who you could discuss your options with. Good luck with it!