Landlord selling apartment

We are in the first year of our nine year lease. Our landlord emailed the other day saying that he is thinking of selling the apartment. Two days later, his sister, an estate agent, called to ask if she could bring someone around to view the apartment that afternoon. We agreed as we were a bit shocked that it was happening so quickly. I'm trying to find out what our rights are and what kind of notice the landlord should be giving us for selling the apartment and for the viewings. Can anyone help?