congratulations on your pregnancy.
I would suggest that your bank will, in your circumstances, be open to a further rescheduling of your debt for the months you are on maternity leave.
Remember that while you are on maternity leave, you won't have childcare costs for your toddler. When your maternity leave is up, the older child will two and a half - so you will have quite high childcare costs for a year or so. Then your eldest child should qualify for the free scheme that should reduce your costs a bit. In addition, as you won't be using all your tax credits for 2011 (as maternity benefit isn't taxable) you shouldn't have any income tax on your salary for the few remaining months of the year when you return to work.
It would be more beneficial if, your partner was able to use some of your tax credits and lower rate band next year. So, if you are planning to marry anyway, you'd be better off financially to do it this year.
When you get back to work, hopefully, things will be picking up a bit and there will be pay increases. Think about what, exactly, your expected income and outgoings will be over the next two years, work out what you will need to live on and go back to your bank. You still have a job. You are paying them off. It is in their interests to give you another bit of leeway.
I know it is really hard not to worry, but please don't let let your financial situation spoil what should be a happy time. You are going to have a lovely age gap between your children and although it will be tough going, you've a great time ahead of you.
good luck.