I can smell gas
First of all, open all windows and doors to ventilate the property immediately.
Then call the free Gas Emergency number on 0800 111 999 as soon as you possible can. If you’re using a mobile, make sure to call from outside as the signal may cause a spark.
Don’t turn any electrical switches on or off (this includes light bulbs), and make sure the gas supply to any appliance has been turned off.