My understanding is that it's at their discretion. It's their apt till they move out and if they don't want someone else tramping around it that's their preogative.
I think if you explain things nicely to your tenant then you both should be able to agree a time & date. I have it in our lease that we can enter the property for whatever reason once it has been agreed with the tenant beforehand.
You don't give notic as you will not be showing people the hous until your tenants move out. What makes you think they will let you show strangers around their home whilst they are still paying for it?
Permission o enter property in a lease won't work either.