Neglected Land next to my house.

Hi,I bought a cortijo some 25 years ago and rebuilt  it.Next to it is a small piece of land,just big enough for a small garden.We saw my abogado and it belonged to an old Spanish lady,whose family lived in France.My abogado told her to sell it to us,as she couldnt do anything with it,and her family werent interested in it as it was too small.She said she would,but when my abogado asked her for the escritura,she hadnt got one,so my abogado couldnt do anything.I think it would cost her more to get one than the land is worth.My husband has mantained it all these years,and kept it clear of scrub ,to avoid any fire risks in Summer.I think the old lady has died and no one has ever been up.How do I go about putting a legal claim in for it.Any help appreciated .