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BigBurner
As far as I am aware, anyone working on any gas appliance must ensure that the appliance is working correctly and safely after repairing it.
If he is not an RGI how can he do this
He is not working on the gas side. Anyone can switch on an appliance and see if it working. In the case of a boiler, if heat is going to the rads it is OK. If he sees that the flames are odd or whatever, then he turns off and recommends a CORGI.