Technically they only have to offer you the same hours that you leave to go on maternity leave on. So you need to negotiate the different hours. I think it is much better to go to your boss and tell them want you want to do well before you go on maternity leave, then you know exactly what your options are now, and you can keep having discussions with them, rather than asking when it is time to come back. I think if you try to do it, once you are on mat. leave it is much easier for them to say no, as you are not there in their face.