People often tell us they have no idea what they are paying for when they book a boiler service. It is a fair question, and the honest answer is that a proper service is mostly checking, not fixing. If the engineer finds nothing wrong, that is the service working as intended.
The parts you can see
An engineer will open the boiler up and look at the components that do the work — the burner, the heat exchanger, the seals. They clean what needs cleaning and look for the early signs of wear: corrosion, soot, staining, anything that suggests the boiler is not burning cleanly.
The parts you cannot
The more important half of the job is measurement. Combustion is checked with an analyser, which tells the engineer whether the boiler is burning correctly and safely. Pressures are checked. Safety devices are tested to confirm they actually shut the boiler down when they should.
This is the part that matters most and the part you will never notice, because a boiler that is burning badly can look completely normal from the outside.
What you should get afterwards
You should be told plainly what was found, what was done, and whether anything needs watching. If a part is wearing out but has life left in it, you should hear that too, rather than being told everything is perfect and then having it fail in January.
You should also get documentation. If you are a landlord, or if you ever sell the house, that paperwork matters.
How often
Once a year is the standard answer, and it is the right one for most homes. Manufacturers generally require annual servicing to keep a warranty valid, so if your boiler is still under warranty, skipping a year can be an expensive saving.