Mouldycheesefan the only reason I'm being allowed to do part time temporarily is because I can't leave before I've been back 12 weeks, not to be reasonable.
If that's true, and I'm presuming you need to stay 12 weeks because of enhanced maternity pay? Then combined with the initial rejection, I'd expect that it's not going to be a positive response and they'll expect you to be full time in October. I would start planning for that - look for other part-time jobs, for example, so you will have to do the minimal amount of full-time work before moving on.
You could approach your GP about being signed off. That is a reasonable course of action in some cases (and as you've seen from other threads, it would then be reasonable to live whilst signed off, as long as it's within reason!). There are issues with that, though. Doctors are under pressure to request adjustments rather than sign people off completely, so the doctor may say that you're able to work less hours or with some home working, or regular breaks, etc. That could just prolong your limbo. Being signed off would also affect your chances of getting a new job, rightly or wrongly, and part-time jobs are already like hens teeth, so you could find having been signed off so recently counts against you and makes it harder to move away from your current job.
I'm bemused, do some of you have six figure salaries and stay at home nannies so you don't mind going to work all day every day?!?
No, but for some people it's not an option not to work full-time. It sounds like you can afford to not need to. Could you afford to not work at all for a period?
It's up to you, at the end of the day, but I'd assess your decision carefully. If they've treated you awfully, you don't owe them any loyalty beyond these 12 weeks, but it seems that you'll be in a much better position all round if you just leave and find another job (or not, if you can afford not to and don't want to work).