I was in a similar situation when I fell pregnant with my first, I had just changed jobs. My old one had paid maternity, my new one did not. Thankfully they would have given me maternity leave held my job for me, but were unwilling to offer part-time, so I chose to resign. I found resigning meant there was a weight lifted off me, as I did not want to be stressing my whole time on maternity leave about whether to go back or not. I did get my job offered back to me as my replacement quit, but once again they were not willing to offer part-time.

So in your case, there is no legal requirement to keep your job open or offer any paid maternity leave. I would probably not push the leave thing just yet, you are getting to know your new job and they are getting to know you. You might find you are happy to never go back once you do go on leave or it may be that you do a great job and they will do what they can to keep you. At DH's work that just happened, a new staff member fell pregnant too early to receive anything, the head of the Dept wanted to keep her so ordered HR to pay her maternity leave and hold her job.

Hope it all works out for the best for you.