A visual check to ensure your boiler still meets current standards
Boiler fired to identify any working faults
Boiler casing removed to check all main boiler components (burner, heat exchanger, main injector, spark/sensor probe).
Checks to ensure flue terminals are unobstructed and internal flue components are sealed properly
Gas valve adjusted to check that burner is combusting efficiently and properly (normally using a flue gas analyser).
Inside casing surfaces cleaned
Boiler parts cleaned if necessary
Gas tightness test conducted to ensure no leaks
Boiler casing put back on - check that properly sealed
Not all boilers are the same
Though the above checklist should stand you in good stead for most boilers, bear in mind that not all boilers are the same.
For most older conventional systems, for example, the following would apply:
The pump and expansion vessel would not be contained within a conventional boiler. In most cases they would be near the hot water cylinder.
Parts such as the heat exchanger, burner and fan would need to be dismantled and checked. This would not normally be necessary for a modern boiler, unless the flue gas analysis suggests that there is a problem.