Hi phelb. This is a very difficult situation your parents find themselves in. Do you know or can you say how much of the 43,000 is arrears and at what stage is it with the bank i.e. are they ready to go down the enforecement route or are they a bit away from that yet. In any event, regardless of their age, this would appear to be no different to what many people are facing on a daily basis. Your parents will need to complete a standard financial statement listing all of their income and all of their expenses and the bank will review this against their standard expenses for a household of that type. The SFS will tell exactly how much money is available to pay off against the mortgage every month.
However, the big issue is for the bank is how long will it take to get their money back and I suspect your parents age will come into play here. If the repayment level is such that it will take 10 years to repay the 43,000 then the bank will be concerned that by that time your parents will be 73 & 77 which then raises a questio nabout their ability to continue to earn as far as then.
I hope that's not too convoluted...