�20 pw sounds about right but i think that once you start paying for childcare (as long as with registered childminder) this will then increase.
You will also get child benefit at around �17 per week.
Thats pretty much all youll get after your maternity pay stops.
Have a word with your mortgage lender and see if you can go to an interest only payment or take a payment break at all to ease the burden, then start looking at ways to cut back. Baby doesnt need much, somewhere warm to live and sleep, food, clothing and love. It doesnt need to cost much altough i think many of us are guilty of buying more than we need.