In respect of servicing, the conventional (non condensing) boilers are stripped and cleaned, condensing boilers are not to be touched inside the heat exchanger for 3 years, the way to check these is by testing the gas pressure (must be 18 millibar) and the flue gas/air ratio must be as per manufacturers instructions. Just as important is the water quality, we test each service to determine water quality, this is because poor system water = short lifespan of the boiler. As explained on the website, many engineers don't have the equipment or capability to service condensing boilers, I was recently told by 3 'engineers' that they service modern boilers, 'the old way', this is impossible.