Just posting quickly as got to run, but I had same problem.
Communal area was enclosed, and the other residents smoke was stinking my flat out, it must have been seeping through my letterbox, and was wafting in from communal area when I opened my front door.
I got legal advice and was told that it was the responsibility of my landlord to prevent noxious fumes from entering my home so I could live in peace, or something along those lines.
Landlord agreed to put up 'No Smoking' signs, but admitted they were not prepared to uphold the signs, as this would mean installing CCTV camera's to catch the culprits who were walking through the communal area smoking, and take steps against those individuals.
Suffice to say I was not happy with this, as it seemed to be a smokescreen (excuse the pun) and I was put on the transfer list for a house.
I have now been living smoke free in my house for a few years.
Incidentally, the landlord has now deemed all communal areas to be smokefree under the ban on smoking in a workplace (cleaners would come to hoover and clean the communal area's), but I was allocated my house before they were prepared to take this step.