Carbon monoxide is especially dangerous because it first produces sleepiness, which is often put down to tiredness.  Once asleep, the effects can continue until fatal. For this reason, compulsory annual testing was brought in by the U.K. government for all rented properties, as the responsibility of the landlord.

All persons who work on gas installations must be recognised as competent by the Health and Safety Executive.  In practice, this means they must be registered with Corgi.

This is not in itself sufficient for testing gas appliances.  Additional qualifications have to be obtained for each category of appliance, ie boilers, fires, water heaters, cookers, laundry, warm air and LPG.  You are obliged to check the back of the CORGI membership card which lists the categories each holder is qualified to work on or inspect.