Sponix wrote:

Would it really matter as long as it wasn't enough to seriously affect
the gas/air ratio? Surely gas is regularly tested to check the BTU/m^3
rating anyway?

Dunno. Who tests it?

There's an adjustment/fiddle factor on the gas bills which is varied
according to the calorific value of the gas. I suppose this should
reflect different ratios of North Sea/Russian gas. If someone had been
able to introduce, say, nitrogen, Transco would soon be aware of it.
I'd imagine that bit would also show up on the combustion efficiency
testers carried by Corgi technicians, so it wouldn't stay secret for
long. An urban myth, I'd think.