I think a basic package would be 6 weeks at 90% pay, then SMP until 9 months, then another 3 months unpaid, and I believe that they have to offer you your exact job if you return within 6 months, and an equivalent job if you return within the second 6 months.
I've recently had a brief catch up about my maternity leave and some of the things covered were...
They want 28 days notice of intention to go on maternity leave (though within reason i.e. if you had planned to work up to 28 weeks but suddenly felt bad at 36 weeks, there might not be time to give 28 days notice)
Ideally a few months notice of intention to return, but I'm not sure on the statutory guidelines for this one. I suppose it would depend on how much you like your work and how open you think they will be about it. Also, they would prefer a few months notice to discuss a flexible working request.
Benefits - if you have any benefits from work i.e. season ticket loan, private healthcare, how will this be covered? (I think this has something to do with not being able to make deductions from SMP, but equally not being able to withdraw benefits)
Childcare vouchers - if you already claim these for a previous child, will they continue to pay during your maternity leave
Pension payments - again, how will they be paid during maternity leave?
Are your holidays accrued during leave, or paid, and if accrued, can you take add them on to the end of your maternity leave. Ditto bank holidays.
You're normally entitled to 10 Keeping in Touch (KIT) days at work, which are paid, so ask about them - will they only pay full days, or half days?
And just double check about your holidays before your leave date. I.e. if you have 25 days holiday in 2016 and were planning on leaving in mid-November, can you take all 25 days leave beforehand? (You should be able to).
If you want to PM me I work in law (though not employment law, as you can probably tell!) and can send you some step-by-step guidance that might help?