We don’t have to tell you how dangerous an un-serviced gas appliance can be. The awful stories you see in the press regarding carbon monoxide poisoning fatalities are usually always down to a faulty or un-serviced gas appliance.
It is extremely important that all gas appliances are serviced at least once a year, even if your boiler appears to be working fine. You would not consider driving a car without an MOT, the same ethos should apply to gas boilers, as both are equally as lethal if not checked once a year and deemed safe to use by a professional engineer.