A proper boiler service follows the manufacturer's schedule for that appliance, and the difference between a good one and a bad one is whether the casing came off. A visual look and a flue reading takes ten minutes and is not a service, however confident the invoice sounds.
The checks an engineer makes
Controls and safety devices tested, the burner, heat exchanger and electrodes inspected and cleaned as needed, seals checked, the flue and its terminal examined, gas pressure and combustion measured with an analyser, and the condensate and the system pressure checked. Findings written down.
What is not in a service
Parts, repairs, a power flush, radiator balancing and system upgrades. Bleeding radiators is a system job rather than an appliance one, and while many engineers will do it as a courtesy, it is not part of the service and should not be assumed.
Boiler service and repair: how they differ
A service is planned maintenance at a fixed price. A repair is diagnosis and parts at an hourly or fixed fee. Where a service uncovers a fault, expect a separate quote rather than the work being absorbed.
What you should be left with
A written service record naming the appliance, the checks made, the readings taken and anything found. Manufacturers ask for it on a warranty claim, it is what a buyer's surveyor will look for, and on a let property it sits beside the safety record in the file the landlord has to be able to produce.
Other questions people ask
Does a boiler service include bleeding radiators?
Not as a rule. Bleeding radiators is a heating system task rather than part of the appliance service, though many engineers will do it if asked while they are there.
How is a boiler inspection different from a service?
An inspection reports on condition and safety. A service is maintenance to the manufacturer's schedule. A landlord's annual check is an inspection, not a service.
What if the engineer finds a fault?
They should tell you what it is, how urgent it is and what a repair would cost. An appliance classed as immediately dangerous will be turned off with permission.
On a gas safety check do they check radiators?
No. The check covers the gas appliances, their flues and the gas pipework, not the wet side of the system. Radiators balancing badly or running cool at the bottom is a service or a system question, which is one reason the two are usually bought in the same visit.