I am not quite clear as to what you mean by "my aunt's name has been put on land registry info to inherit the estate." This could mean multiple things
from your mother being added as a joint owner of the property to your grandfather establishing a life estate. The precise repercussions depend upon exactly what your grandfather did. A promise to make a gift of property in a Will as you indicate your grandmother told your father is not enforceable. If your grandfather was not competent at the time that he altered the title to his property, any changes he made would not be valid. If, your grandfather is competent, you may suggest that he contact his lawyer to make sure that his estate plan comports with his wishes. If you believe he is not competent, you should contact a lawyer and look into establishing a guardianship of him and then you would be in a position to challenge whatever conveyances of property interests he may have done when he was not competent.