I know an engineer who only services the burner, never takes baffles out or checks the tank filter - to me thats like having your car serviced and them only checking the oil levels and tyres pressures.
When I service it consists of:
BOILER:
All baffles out and wire brushed to get most deposits off. Sometimes I have the get the angle grinder on them if they were tight to remove (Grants).
Wire brush the inside of the boiler.
If a condensing unit, pull out all the turbulates and 'rod brush' the tubes (process depends on manufacturer).
If condensing, clean trap and re-prime with water.
Note, advise, tackle any issues, leaks etc
Consider the condition of the insides, this can tell you alot about the combustion history.
BURNER:
Remove blast tube (wire brush inside and out)
Brush electrodes (before removing old nozzle) and check for damage if ceramic. Look for any signs of arcing from the electrodes onto the nozzle or the blast tube.
Replace nozzle (I do every year, some dont)
Check flexible oil line, replace if needed.
Check fan and airbox for debris/ fluff
Check/ clean photocell
OUTSIDE CASE:
Visually look over any visible oil lines, unions, valves etc.
Look for fire valve and if Teddington type, pull the switch to see if feels ok, if no valve, make a note on service sheet and maybe advise to fit one if appropriate.
Look for oil filters in line and by the tank. If present, clean strainer/ replace element or if absent, advise installation of one.
I always spray the tank valve assembly with WD40 too.
Look over the tank and report any concerns.
But I charge £90 for a condensing plus parts so sometimes the area dictates the price, sometimes its the quality of the service. I assume that I will look after the boiler for a long time so will always do a proper and thorough job.
Worth noting that alot of engineers in this area (not there are many) are not capable of simple fixes like AAVs, diverter valves, plate heat exchanger replacements. Its a good thing to find another engineer in your area who is capable of these things and when required, ask him to do the work but attend and learn.