AAhh! I just wrote a long reply, but lost it!
Maternity pay depends on your employer, but is likely to included some element of Statutory Maternity Pay, which is about £125/week. Check out Direct Gov and Entitled To websites for details. There is also Child Benefit, Working Tax Credits and Child Tax Credits.
I will be going on maternity leave with my second child at the end of the month. First chidl was not the financial shocker I thought he might be, even though I went back part time and lost 40% of my salary straight off. When I was on maternity leave with him, I arranged just to pay the interest on the mortgage and now overpay when we can to redress the lower repayment repayments. I am not sure if you can get Housing Benefit when you are on SMP if you are renting.
DS (3 yesterday) doesn't not really want for anything, and hand me downs, eBay, Mumsnet, NCT Nearly New Sales and Charity shops make life easier.
We don't go out half as much as we used to, and after one trip, have decided that foreign hols are not worth the effort (wimps!) so have holidayed in England and Wales mostly in the last 3 years.
i actually still manage to save a bit and don't have any debts apart from the mortgage. We do have a very low key lifestyle though, but that is deliberate.
If it helps you compare you situation to mine, which is manageable, our joint income is about £55K/annum (before tax), and outgoings are about £1700/month now, including childcare for 3 days a week.
Don't think about the numbers - just do it - everything else will fall into place.
Good luck 