You need a magnetic or stick on warning label which is a green diamond with a bottle icon on it, this will have "compressed gas" and a "2" at the bottom. It is an offence not to show this if compressed gas is being carried in the vehicle. It is advisable to have one on the front door of your home or the room in which the bottle is kept. I'd advise magnetic because if you do want to become one of the " just do it" brigade then you can remove it before going through road tunnels, which is also an offence without an escort.