I know that property law is different in England & Scotland but i'm sure the basis is the same - if the deeds & mortgage are in the son's name then that's that unfortunately. I'm unsure from your post as to whether your OH's gran is currently living in a mortgaged property and is contributing towards the property but if she is perhaps try proving that she's contributed to the payment of the mortgage (as in matrimonial cases where one spouse contributes to bills, mortgage etc but is not on the deeds) and see if she can legally obtain some form of beneficial interest in the property?
If i were you i'd obtain some legal advice on her behalf but it sounds like a very sticky situation. Good luck xxx