Hi Bugsy, I'm currently of the opinion that anyone wanting to do a PhD is a little bit bonkers, but that may just be the stress of my latest deadline talking 
1. is there any point doing a PhD in social research if you aren't going to pursue an academic career?
I don't really know enough about this area to say. You may not have to move around the country, how many universities are there that are commutable from where you currently live?
2. do you usually get maternity leave / pay on a PhD?
It depends on the conditions of your funding. The local body that funds the majority of PhDs in my university have a maternity provision of a full 6 months funding for the one baby. I don't get this funding but as they had previously provided another student who was also not funded by this body with this maternity provision I was given paid maternity leave.
However, I'm now pregnant with my second and although there will be no issue with me taking the leave I won't get any money.
As you don't pay tax or national insurance on the stipend there is no automatic entitlement to SMP or maternity allowance. Although if you are earning extra money through teaching/ marking/ invigilating etc. and depending on the amount of time elapsed from your previous job you may still fit some of the criteria for maternity allowance. You can check this out here
www.direct.gov.uk/en/moneytaxandbenefits/benefitstaxcreditsandothersupport/expectingorbringingupchildren/dg_10018869
3. Should I ask the supervisor about it, or wait until I am offered a place?
Not sure if you are asking whether you should ask about maternity leave or ask about the application in general.
I would ask about the application in general, but try and find out about maternity provision of the funding via a websearch. The local body funding contract is available on our university website and other funding bodies may also post their full terms.
As others have said though there should be no problem with you taking a temporary withdrawal for maternity leave.
4. Is the idea of doing a PhD while pregnant and with a small baby completely insane? Has anyone here done it and what was it like?
Yes it's insane, but at the same time it also makes perfect sense for me as I'm quite old and am really glad that I'll have baby making out of the way by the time I finish and need to get a proper job.
The hardest bit in my first pregnancy was the tiredness in the first few months, the hardest part this time has been the tiredness in the first few months and having a baby who wasn't sleeping very well. I really wish I had been able to get a few months work done before being pregnant again.
I also find that pregnancy really affects my ability to concentrate.
The hardest thing about having a baby is having to fit my working schedule around his schedule. I used to come in to the office mid-morning and work till about 8, I now come in very early and have to leave at about 4.45, which always feels like the time when I'm just hitting my stride. I'm also too shattered to work late into the night to meet deadlines, but I think that is a lot to do with pregnancy rather than the DS.
5. If I was pregnant by September, would they still let me start the course? Do you think there would be an option to defer for a year?
I don't think they could stop you, they wouldn't be able to stop a man about to have a child from starting. Whether or not you think it would be wise to defer would depend on how pregnant you were.
HTH
Good luck with the application.