Grouch wrote:
Whoever installed it should have checked and advised what ventilation was
required; if they didn't I'd be very suspicious as to their qualifications
to do such a job.

The fitter did check, advised us 100cm3. The floorboards that were
down at the time a) had up to 4mm gaps between them, and b) had a hole
about 10cm x 10cm where a first, aborted, attempt had been made to fit
the fire by another fitter so there was more than enough ventilation.
There was no point in him insisting on a vent being added because he
knew we were about to lay a secondary floor which would change the
situation.

Hence my question:
1) in a draghty house is it still necessary (not to comply with the
regs but to be safe)
2) where can i buy a suitable vent? (and if they're measured by
external diameter rather than cubic airflow what size vent do i need?)