Smoking in an Apartment/Rental

I smoke in our rented flat. Our landlord, who is a non-smoker, does not mind, as he will have to repaint the flat and change the old carpets in any event before the next tenants would be moving in, and neither does the couple living in the flat above us, of whom the husband smokes. However, I keep two windows slightly open at all times... Luckily, UK legislation cannot (yet) forbid smoking in rented flats, provided you have the landlord's permission.