The message
from "Roger Mills" contains these words:

My in-laws' bungalow (built in about 1960) had solid floors with
under-floor
ventillation for the open fire built in. ISTR that there was an airbrick in
an outside wall, with a metal duct below the screed - terminating under the
fireplace. I think the relevant bits were made by Baxi - but I doubt
whether
they make them now!

Yes, of course Baxi still make underfloor-draught fireplaces and
necessary bits.