There was gas leaking out of my gas riser room outside my flat. I have verified it is not from my cooker as there are no gas smell in the kitchen.
I have no alternative but to activate City Gas in the evening. The service team came one hour after the activation. They check with a gas meter and verified it was not from my piping. Their suspicion is from the flat below. The town gas is lighter and will rise up through the ventilation provided in the riser room.
Thus they went to the flat below us to conduct their investigation. The City Gas never update me and I leave it in their good hands.