I'm no expert, but I'm a landlord myself. Albeit a nice one!
Your landlord is allowed to show the property for sale as long as you are within notice period. That said, he must give proper warning to you of each viewing and they must, of course be accompanied by the estate agent. As a tenant you are expected to make allowances for this and keep the place generally in a viewable state. You could request (as anyone would) that viewings are conducted during office hours (if you work) and that they group together viewings at the weekend, so that you are inconvenienced in the smallest way possible. Just politeness really.
Not ideal at all, but unfortunately just the way it is.
Estate agents - pls correct me if I'm wrong!!
Good luck.