Thanks for the replies.
Part time wouldn't work in terms of the shifts, they are pretty much non negotiable and can't be split into shorter shifts. I work on a rota with 2 other members of staff and we are the only one working when we on shift if that makes sense.
My husband is of course going to help with arranging child care but the issue is that he starts at 6:30am most days and DS often wakes at 6 these days so I'd have to be up with him then, after minimal sleep. He's a pretty good sleeper at the moment so I'm not up lots through the night but not getting to bed til after 2am is the issue. Yes sleep deprivation is normal with a baby. But I am a nurse and being the only one on shift expected to deal with emergencies if required, I wouldn't want to risk being too tired and making a mistake, being up with a baby isn't something I can help if I am but working these new hours would just be adding to that.
I wasn't consulted on this - it was a business change (non NHS) and it has changed for us all and I do appreciate it is an unavoidable change due to wider changes that have occurred due to the pandemic unfortunately. I guess if I hadn't been on mat leave when it happened, I'd be in the same position and have to leave too and it's just unsuitable for me and not the shifts I signed up for. Had they been these hours from the start, I wouldn't have wanted the job.
My husband is unable to work flexibly but his hours mean he is home every evening which works well for us. I'm lucky that my parents are my main childcare too. They have declined my request for day shifts only - I offered to work more days too thinking I was being reasonable and giving them options but apparently not! If I were to return to the new shifts, it would be mon, wed and thurs only. But I wanted to go part time to have more time with my baby, not to be tired and grouchy as I know I would be 
Basically I know now I won't be returning. It's reassuring to know a few of you think it's unfair for them to reclaim the company maternity pay, this is my main concern really. I'm waiting for response from citizens advice and maternity action with regard to how to approach it with some backing.
Thank you all again, I hope I've answered everything