Congratulations, hope all goes well with the baby.
As maternity benefit is not taxable your wife will be due a tax refund at the end of the year. Ask her employer to give do up her P60 in the first week of January (it's the least he can do) and send that off to your tax office ASAP afterwards to get the refund due to her.
I'd try talking to the mortgage company again, even just to reduce payments for a while. You are going to have a permanent pensionable job very soon, you'd imagine they'd give you some leeway.
If you still have to pay the mortgage, things will be very tight. Would getting a loan be an option? If you get a flexible loan over a long period, say 5 years, the repayments will be low enough and you can repay it early when you are both earning proper money again.
In the meantime, try to go easy on the baby purchases. Family and friends will get you more than you could imagine (particularly if you let them know what you need, when they ask).