Hi Jen
I'm not 100% sure on this, but this is the way I think it works. If you work full time and are entitled to maternity leave then they have to give you a job for same money/hours if you want to go back. If there is nothing for you, then they would have to offer you a redundancy.
However, if you worked full time and want to go back part time, they don't have to agree to that and you would have to resign if you didn't want to go back to your old hours. They don't have to give you a job with different hours/conditions than you had before.
This is the way it worked it the last job I was in and from speaking to others, seems to be pretty standard.