Call the National Grid gas emergency line on 0800 111 999 straightaway.
Then follow these steps:
put out any naked flames, and don’t smoke
don’t use any electrical switches. This includes light switches and battery-operated items (if you have any electrical/battery operated security entry systems be careful not to operate them when the engineer arrives)
contact the fire brigade if there is a strong smell of gas
leave the building immediately.
If you think you have a faulty gas appliance
Call a gas-safe registered engineer. By law, gas engineers must be on the Gas Safe Register. Always ask to see their ID card as this will show their registration details along with the gas works they’re qualified to carry out.)