Smell of Smoke from Below... How do I fix?

Shoot the smoker downstairs

Erm, whoever monitors these answers look at the other reply! I have had answers declined for a lot less than the other answer...which is actually a rather poor attempt at a joke, rather than an answer.

You can’t argue it wouldn’t solve the problem...

Some older properties have a common cavity ducting system interconnecting floors using air brick systems; perhaps checking you haven’t got something similar partially blocked/ obscured from view first may be a good step? Assuming your floor covering is a vinyl, a simple approach would be to remove all the furniture from the space, remove the floor covering, and lay a DPM polythene sheet over the entire area, before sealing at every edge with a sanitary silicone sealant. Carefully cover the area with hardboard, taking care not to damage the underlying DPM membrane, and simply re-fit your sanitary ware and floor covering. To be honest, if it’s that bad you would probably just push the problem under the floor to another room, and the only way to address it properly would be to work with your neighbour below to find, locate and rectify the point at which your neighbour’s smoke is leaving his property.