hoursandhours you are right you do have additional protection as you are on maternity leave. If there is a suitable position for you you must be offered it without having to compete.
However a suitable position would be one on your existing terms and conditions, not on terms and conditions you would like in the future, so making it clear you want different hours will jeopardise your extra protection.
Unless you feel reduced hours will be more likely to give you a job than not (highly unlikely obviously), don't muddy the waters with a request at this point.
Make them honour their obligations to you, make sure you get allocated a suitable post and get it confirmed.
Then at a suitable point when you are thinking about coming back, put in a flexible working request as you normally would based on the new job you have been allocated.
On the other hand what you could do is write stating that you understand as a woman on maternity leave you have extra protection and the right to be offered a suitable position as a priority above other employees where there is one available. Say that you are sure they are aware of that and fully prepared to honour their obligations in this regard, however you would like to also put them on notice that you may also be prepared to consider part time alternatives.
That way you are reserving your right to a full time position if there is one, but also making sure if there are any part time positions you get offered them as well.
If you do decide to go that route, I'd only do so if you have reason to believe there actually will be some part time posts, and definitely don't phrase it as a request on your part, more as a 'I'd be prepared to consider it to help you guys out.'
Up to you. I'd probably just get myself assigned a full time suitable post and worry about reduced hours later tbh. Safer that way, you keep the protection you are entitled to which gives you less stress at a time you really don't need it. Also that way when you do come to make your flexible working request, you will know more about what kind of part time hours you want to fit in with your childcare arrangements with how you are feeling at the time rather than feeling rushed into an arrangement too early.