Emergency Gas Engineer
If you have a safety issue with an appliance the first sensible thing to do would be to shut down the unit and then isolate it from its power source there is an electrical isolation switch on all boilers, if there isn't one on your system then you should have on fitted straight away. Once you have carried out the isolation of the appliance you will be safe until a qualified engineer can inspect and ID the problem, if you suspect a gas leak then you should immediately shut down the Emergency Control Valve this is usually adjacent to the meter, in the unlikely event that the ECV has become jammed you should contact Transco on 0800 111 999 and advise you have a gas leak and you cannot isolate the the supply and the situation is immediately dangerous.
During this process you will need to open all doors and windows to ventilate the gas from the building, one advantage of NG is that it is lighter than air and will dissipate naturally. When Transco arrive they have the option of shutting down the supply externally from the building.
If you are ever faced with this situation please feel free to contact us for immediate advice on 01843 223458