I don't know the actual law or your tenancy agreement of course. But a request to allow a potential purchaser access the the property for viewing, with several days notice, seems entirely reasonable to me. I'm with the "why be a nob?" brigade. Unless relationships are already strained or there will be an absolute flood of people then I don't see the point of denying a reasonable request.
	
		
			
		
		
	
				
			 
	 
  
 
		 
 
		 
 
		 
 
		
 
 
		 
 
		