Can't avoid smoke

I have what I would call very mild asthma, probably lifelong, but only diagnosed a couple of years ago. From an early age I would get a coughing fit if I was in the same room/house as a smoker, so it's fair to say this is probably my biggest trigger.

I was over the moon when the public places smoking ban came in, but I still suffer from smoke getting into my flat from neighbours downstairs, who smoke heavily. I didn't think smoking was allowed in the flats when I moved in, but at least two neighbours smoke, and this ends up in the corridors and even comes through the walls at times when its bad weather and the neighbours have their windows shut..

It's not a public place as such.....do I have any rights in asking the landlord or letting agent to do something about the smoke?

From experience, very few rented properties I've come across specify 'no smoking'.