So presumably in old, draughty churches without heating it must be impossible to get accurate tuning for the organ for the same reason.

It's pretty difficult to get accurate tuning on any kind of church organ in any case...

True. Accurate was the wrong word... consistent?

(Darren) That's correct. The whole instrument would be flat and it wouldn't sound wrong but any other instrument accompanying it would have to tune down a bit. Rather difficult with a piano. One way round it would be to have a small bleed of a lighter gas into the air the organ uses. You could use hydrogen, helium, neon, methane, ammonia or hydrogen fluoride. Perhaps I should patent this lunacy.

Wouldn't some of those risk producing the Amazing Exploding Organ? That puts me in mind of the Large Hot Pipe Organ which produces sound by exploding a propane/air mix in its pipes.