Mortgaged land beofre independence.

Hi Sir, My self Rajendran from Karnataka. We have purchased a land in the year 1980s and built the house in 1996. and we have paid all the years tax correctly. But we have received a lawyer notice to entire area nearly to 30 houses. They have mentioned in the notice. that the land was belong to my grandfather who have mortgaged the land to xy person in 1942 for the amount 200rs. we dint sold any site for any one only our grandfather has mortgaged the land in 1942 for 200rs. without to our knowledge you have builted the houses. My Question is here We have purchased the above mentioned land from 4th party after Independence, in 1980s and builted the house in 1996, but now we received this type of problem. but we replied to the lawyer notice we have purchased the land from xy party and we are paying all the tax for house without any arrears we are ready to take any legal action. Now they went for court and we awaiting for the date. but they have only mortgaged agreement. there is no any other document. please send some valuable advice to my mail. Regards Rajendran