The first thing is to consider the source of the smell. Smoke can arrive by any number of methods including through open windows, down neighbouring flues and through leaking flues. First ascertain where the smell is strongest, is the smell there only when the fire is lit. Is there always a smell upstairs when the fire is lit or just when the wind is in a certain direction. With the fire lit, go outside and watch the behaviour of the smoke as it leaves the pot. Does any appear to be drawn down neighbouring pots?
If there are no clear signs of the cause, get the flue swept and if the problem persists then it is likely that the flue is leaking and will need lining. A CCTV survey may confirm this.
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