On Sat, 30 Jun 2007 21:37:46 +0100, Mike D
wrote:

Several people (including myself) have noticed a slight smell of gas
in my house,

Gas isn't meant to leak. If it is leaking, it's likely to be either
significant enough to be a risk of explosion, or else it's coming from
the exhaust of the boiler / gas fire not being vented properly. The
second of these can also have a risk of CO along with it, and low levels
of that can certainly give the symptoms you describe.