On 30 Jun, 21:37, Mike D wrote:

If the tiredness is a symptom of carbon monoxide poisoning, I believe
a blood test can confirm that.
If you see your GP, mention your suspicion.

My house is about 100 years old and has various old redundant gas
pipes, imbedded in the walls and under the floors, but the only pipe
which is supposed to be live is the one going directly from the meter
to my central heating boiler. And that is all at thoe opposite end of
the house from where the slight gas smell has been commented on.

Also the service pipe from gas main to meter, which they can't test
with the usual pressure-loss test.
Possibly it could be from a gas system next door, or gas mains in the
street; less likely but it has happened.

BG, Transco ( or whatever they're called now) I've found to be utterly
useless; they left a leak from their gas service pipes or mains
(which they had known to be corroded) into my house, until I'd
replaced my gas installation. Then they lied through their teeth about
it. They change their name when the bad publicity accumulates to an
unacceptable level.

BTW, is natural gas heavier or lighter than air?

I'm fairly sure it's lighter, but small leaks usually gets dispersed
by air movements & convection rather than forming a layer on the
ceiling.
Propane and butane are heavier.