In answering I assume the will you refer to left the property to someone other than the two people now living in the house. Also you state the will wasnt executed , but I assume you mean it wasnt probated.
Generally you cannot claim ownership of a property left in a will until 7 years after the death- if the person entitled was in their right mind, and wasnt a minor.
After 7 years you can begin to go into adverse possession. So if the death was more than 19 years ago, it is possible that this couple could claim adverse possession. That is if the other person who tried to sort out the title didnt remain there until recently- i.e if they left more than 12 years ago. And if they werent in the property by consent.
This is all very general, but the particular circumstances may negate any of the above. Really they need to talk to a solicitor and go through the history of the property, the family tree, what wills were left, who died without a will etc etc