Rights of a tenant

Hello Sir/Madam, We live in a rented flat in Kolkata since 40 years.The rent was being paid in the name of my grandfather.After his demise, rent is paid by the name of my father and grandmother @Rs.650/month.The land on which the building is built had been taken on lease by the landlord for a period of 99 years.Still 10 years are left of the lease period.To inform you all the expenses starting from repairs,water bill and all other expenses related to the flat are borne by us.The landlord does not even visit the property.My grandfather had sent many letters to the landlord regarding repairs of the detiriorating condition of the flat but the landlord did not care.The rent agreement between the landlord and my grandfather does not bear any clause regarding the period of rent or any periodical increment in rent.I wanted to know about our rights being a tenant of this flat since 40 years. Can the landlord force us to leave the flat? Also if we wanted to buy the flat can it be done? If my grandmother dies,can the landlord force us to leave the flat? We have heard that the landlord is trying to sell this flat to some other person.I will be very grateful to you if you could inform me about our rights a s tenant.Please help. Thanks in Advance