well if it's a government council they have their own mat policies and will pay you a certain sum over your mat leave - have a look at the relevant funding council's website?. As a university if we are dealing with a self funding student I believe we offer deferral but no mat leave pay as it were, you just take leave of absence. But need to check <goes off to check>. Not sure about the situation if fees are covered by an internal bursary.
I had research council funded PhD: when I went on leave my funding was interrupted and then restarted (the clock had been stopped) when my supervisor confirmed I had returned. I don't think I had any additional pay during my leave.
From the point of view of the university (who wouldn't normally be in the position of an employer as studentships are not employment contracts) as far as I know this would be treated like any other interruption for personal reasons; you interrupt at a certain date and return as required (normally a year later). You'd need to be careful to not exceed the university's maximum registration time, but this is typically quite long.
Do check local regulations though as the can differ quite dramatically between universities.
I think it depends whether you are "just" doing your PhD, or whether you are also teaching for the institution. One of my friends has just had a baby in the middle of hers and she certainly got maternity pay, but she was definitely teaching as well as studying IYSWIM.
Don't forget that in certain subjects at least, the field may have moved on quite substantially during your maternity leave. May just mean you have to keep up while you're on leave, or spend a month or so catching up when you return, but at worst it can mean that someone's already scooped your PhD and you need to rethink. A friend did it, and said it was ok, but that she really needed to keep abreast of the field and make sure people were aware of the area she was working on.
I think you have to give birth within the period of the grant though (rather than the government who look at whether you are in work at around 5 - 6 months). They then extend your time by the amount you take off.
I was full time and submitted 48 months after starting. I was funded for 46 of these months esentially (although of course 10 months I was suspended). You usually can have up to 42 months candidature can't you full time .
I was self-funded and just took a year's suspension of studies for my first child. Submitted four days before my second child was born. Viva at him being 9 weeks (top tip: if you mention you want breastfeeding breaks they do your viva very swiftly!!)
I have been told the AHRC would give me something money-wise, but (despite my supervisor telling me it's a good time to have a baby) I haven't investigated as DH doesn't want to yet. They would certainly stop the clock for me, I know that. However, a lot of people keep telling me it doesn't get any easier after the PhD.