Modern (last 15-20 years) boilers have very few serviceable parts, if one part breaks or fails the gas valve is auto shut (legal requirement safety feature) if the boiler over pressures or burner pressure is over acceptable limits the gas valve auto shuts off...
These Legally required features have been in place long before that, even on instant heat water heaters.
Boilers from the last 5 years have far more control mechanisms than ever before, including cO2 monitoring external to the device than ever before.
As a Gas engineer (service and mainline supply) I would not pay for an annual service unless the boiler was 15+ years old or I lived in a very hard water area.
These Legally required features have been in place long before that, even on instant heat water heaters.
Boilers from the last 5 years have far more control mechanisms than ever before, including cO2 monitoring external to the device than ever before.
As a Gas engineer (service and mainline supply) I would not pay for an annual service unless the boiler was 15+ years old or I lived in a very hard water area.