Servicing boilers is a thing of the past since natural gas emerged. The reason boilers needed servicing originally was to remove all the ash deposits left by coal gas, if you didn't have it cleaned it would soot up, as the flames turned from blue (healthy) to yellow like a candle flame, which coats in soot anything it touches (unhealthy carbon monoxide is given off) Every time some one messes with your boiler, the more chances they have of forgetting to refix what they removed. Explosions are from a build up of gas in a confined space, if you have all your faculties IE sense of smell, then you are not in danger. Whilst the boiler is running let it be, repair it when it stops heating the place, or if it becomes irritatingly noisey that is the heat exchanger that Bob mentioned. I despise these insurance policies that are sold on irrational fear, they are such a rip off and they are usually designed not to pay out, when one is most in need. The most troublesome items in a CH system are the motorized valve, the thermostat, and the pump, each can be replaced more cheaply than your annual insurance layout. The odds are unlikely you'll need 2 items exchanged in the same year. Dear Asset, I really hadn't intended to chew your ears off. Bet you wont be asking me about servicing or anything else again;-)