What does a service involve?
Clean through the burner, pilot and main injectors.
Ensure the correct operation of any thermostats and safety devices.
Change any relevant gaskets and rope seals.
Clean baffles and heat exchangers.
Check gas pressures including burner pressure and gas rating of the appliance.
Check combustion and chimney performance including spillage tests.
Check for general safe operation of the appliance.
Clean and check the operation of any fans.
Cosmetic checks ie. topping up insulation etc.