Smell of gas or a hiss

I can smell gas, or hear a hiss

The manual: shut off the gas, put out any flame and open the lid. If the smell stays, keep away and call your gas supplier or the fire service. A hiss from the cylinder: move away, do not fix it yourself, call the fire service.

Once it is safe: check the regulator is fully on and leak-test every joint. A leaking cylinder valve or regulator needs a new cylinder or Weber.