Ignore the people who are telling you to get a life Burston.
It's really annoying when you can smell something you don't like and can't trace the origin, and where are you persecuting smokers? As far as I can see you are just trying to find out how to stop the smell coming in to your house.
I have a mate who has virtually no sense of smell, and he can never understand why I make such a fuss about certain smells, I don't choose to smell smoking smells, I don't even try to but I can smell cigarette smoke from a looong way off so I know how you feel. Obviously those who are knocking you either have a poor sense of smell or smoke themselves and just don't realise how annoying this is.
We had an intermittent sewage smell years ago (early 70s) which took weeks and weeks to find the source even though my parents had the council in many times. Are the knockers saying you should just ignore this??? I doubt that anybody would ignore any smell that was annoying to themselves.