Hello, congratulations on the pregnancy and so sorry about the stressful situation you find yourself in. Haven't been in the same situation myself but didn't want you to have no replies.
I know you say your ex doesn't want to be involved, but will you be getting child maintenance from him? If not I really think you should reconsider, fair enough if he doesn't want personal contact with the baby but I think it is his moral responsibility to pay at least the CMS minimum to ensure you are able to feed the child he helped to make and put a roof over their head. Do make absolutely sure also you will be claiming any benefits or tax credits you are entitled to (the website entitled to can calculate for you), even if not much due to your income every little helps.
Then perhaps you can do a bit of a long hard look at your outgoings and see if there are any areas you can cut to live within your income both whilst on maternity leave and after returning to work? Have you looked into how much better off you will be after returning once childcare is accounted for (if a lot then you might need to consider taking a shorter maternity leave then the full year which may not be what you'd ideally do but needs must to bring in the money).
Obviously things like gym, holidays, 'frittering', expensive phone contracts etc are easy areas to cut, plus also make sure you are on the cheapest possible rates for utilities and insurances etc. You can often live quite cheaply whilst on maternity leave as you don't have commuting costs etc. - and a lot of baby stuff can be picked up very cheaply or second hand. Perhaps more expensive items or things you shouldn't buy second hand like a car seat or cot mattress you could ask for as gifts from family?
But if you already don't have too much discretionary spending which can be easily cut or your income shortfall is massive you might have to look at more drastic measures, i.e. areas where you spend more. For some people this could mean swapping your car for a smaller/older model (esp if you have it on finance) or getting rid of it altogether, or housing costs - ie moving to a smaller place or a cheaper area. Of course these aren't things you can easily change but the good thing is you are planning in advance, so perhaps worth looking into now - or you could try posting your budget on the MoneySavingExpert forum to see if anyone can advise other ways of how to make the books balance?
Good luck to you, hope it works out 