On 20 Dec, 12:59, tvmo wrote:
Oh..just go round the skirting with some decorators caulk.

That's one solution, but it doesn't exactly solve the problem. In
addition, I block up one exit and the smoke will find another.

Yes, but you'll block the bigger holes so reducing the amount of smoke
ingress.
You could track the holes by generating a negative pressure in one
house (extractor fans on next door, assuming they'll assist, on in
your house if they won't) and going around the skirting with a thing
called a smoke pencil. In the absence of a smoke pencil, a cigarette
will do. Blower doors are increasingly being used in this manner to
achieve air-tightness compliance with Building Regs (commercial stuff
only, I think).

The unused flues are a prime suspect. The acidic compounds in smoke
residues from coal/wood fires corrodes the mortar & also bricks often
get dislodged unnoticed by peeps nailing on skirting boards. If they
have an air vent into their old flues, that may be the main route.