Follow the following safety steps
Open your doors and windows so the gas can escape
Don't smoke or use matches or cigarette lighters
Don't turn light switches or anything electrical on or off, including mobile phones
If you can, turn off your gas supply at the meter and leave it switched off until you're sure it's safe to turn it back on again.
Contact the National Gas Service Emergency Line on 0800 111 999 to report a suspected gas leak.