gem - the best thing to do is go to your local job centre, as they can give you info on all the benefits you'd be entitled to and tell you all the (numerous) forms you need to fill in to get them.
Basically though, if you're a lone mum with very little or no income you should be entitled to;
*Income Support (to make up your income)
*Child Benefit (all parents are entitled to this)
*Child Tax Credit (given to all parents with an income of under about £58k, but it gets higher the lower your income)
*A one-off Maternity Grant of £500 (if you're on Income Support, or certain other benefits, to help you buy baby stuff)
*Milk tokens (given if you get certain benefits, like Income Support. Can be used for milk for you to drink before birth, then for milk for you if you breastfeed or formula for the baby. Given until the child is 5, unless you come off the qualifying benefits.)
Then there's also Working Tax Credit and the childcare allowance of working tax credit which will help you if/when you go back to work after the baby is born.
I think you've until a couple of months after the baby is born to claim these but some of them can't be backdated, or not for very long, so the sooner you claim, the sooner you'll get the benefits and the less you'll miss out on.
Would highly recommend doing it all through the jobcentre - they should have a lone parent advisor to tell you all the details and mine has been fantastic. The tax credits can be a nightmare and he's helped me put in claims and chase them up when I've needed to.
HTH