If you smell gas, call the National Gas Emergency Service on 0800 111 999 immediately. Do not call from a mobile phone as this may create a spark which could ignite the gas. Go outside or use a neighbour's phone.
You should also:
Check the pilot light, then turn all gas appliances off. If the pilot light remains on there may be a gas leak.
Turn the main gas tap off. You can do this by turning the emergency control valve on the pipe leading into the meter to the 'OFF' position. The gas supply is off when the ridged line on the spindle is across the pipe.
Open doors and windows to clear the gas.
Do not use any electrical appliances. This includes turning light switches on or off and using doorbells, mobile phones, or any other electrical switches.
Do not smoke, light a match or use any other kind of naked flame.
Do not leave it or expect someone else to call the national gas helpline - you could be putting yourself and others at risk.