Doug. wrote:

I was told by the experts that the gas emitted from present-day gas
boilers is not in any way toxic or a danger to health.

They are correct.
The only products from a correctly functioning gas boiler are water and CO2.
(Water is the most dangerous statistically.. it kills dozens every year.)



My only solution was to knock down the breakfast room (adjoining the
garage) and rebuild with an illegal wall 8 feet high, an expensive
safety-glass sloping roof and it is now a utility room with
clothes-washer, cupboard, fridge-freezer.

I would have thought that an extractor fan mounted in the window would have
done the trick for about £60.
If you had fitted a really powerfull one his boiler would probably have
failed to work correctly.