... I'm about to put one in the
> hall on the gas, to match the one on the water. (At the moment you have
> to find the key for the gas box outside in order to isolate the gas).
Sounds like a recipe for disaster. Someone turns it off thinking it't the water one, goes "Oh that's not it" and turns it back on again. And then your wok burner ring which was alight before is now pissing out unburnt gas into the kitchen ....
Why not just hang a key for the gas box somewhere convenient?