They need to offer you an employment contract and all of the details will be in that. You can ask for them to go through all of the details in the contract over the phone or to email you a copy of all their employment terms & conditions prior to you recieving the written offer.

Maternity leave is legislated and the minimum requirement is for them to offer 6 weeks unpaid maternity leave. After you have been with them for 12 months then it becomes 12 months unpaid maternity leave and they are required to give you the same or equivalent job back on your return. You are also able to request altered working hours on your return and they are required to consider the request and give that to you if it is reasonable and workable for the business (so they can basically just say no).