These are the stages I do in a standard (annual) boiler service with all Glow worm and Vaillant boilers with the stainless steel heat exchanger.
Drain boiler
re charge expansion vessel
lubricate Schrader valve
remove and clean condensaye trap, automatic air vent , pressure sensor.
the combustion and pressures are checked along with all of the safety checks. A service certificate is filled out detailing the work carried out and readings with advice for the homeowner if required.