There are three separate activities and they are constantly confused. A brief visual check by somebody on site, frequently. A basic service by a competent engineer, annually. An extended service or overhaul at a longer interval that depends on the agent. The statutory hook for all of them is the duty to keep fire fighting equipment maintained in efficient working order and in good repair.
The monthly check, done by you
Somebody on site confirms each unit is in its place, unobstructed, undamaged, with the tamper seal intact, the pressure gauge in the green where fitted, and the instruction label readable and facing out. It takes a few minutes per floor and it is recorded in the fire log book. This is the check that actually finds problems, because it is the only one that happens between engineer visits.
The annual basic service, done by an engineer
A competent person examines each unit against the relevant British Standard, checks weight and pressure, inspects the hose and horn, replaces seals, and signs and dates a label on the unit. Whether the person is competent is the duty holder's judgement, and third party certification of the servicing firm is the usual evidence for it.
The extended service and the pressure test
Water, foam and powder units normally have an extended service at a longer interval where the extinguisher is discharged, examined internally and recharged. Carbon dioxide units instead go for a pressure test as high pressure vessels. The intervals differ by agent, so a building with mixed types has a staggered schedule rather than one date.
What the record has to show
The date, what was done, by whom, and the outcome for each individual unit rather than for the building as a whole. An inspector reading a service certificate that says the site was serviced learns nothing about the unit in front of them; a label on the unit and a schedule listing each one does.
Fire extinguisher checks how often, by who and what they cover
Three checks at three intervals, and only one of them needs a contractor. Monthly, somebody in the building walks round and looks: is each extinguisher in its place, unobstructed and visible, is the pressure gauge in the green, is the seal intact, is there any visible damage, and is the sign there. That takes minutes and is recorded. Annually, a competent technician carries out the basic service, which includes checking the weight, the hose, the horn, the internal condition where the standard requires it, and replacing seals, and they sign the label and the log. At longer intervals the standard adds extended service, commonly every five years for water, foam and powder units, which involves discharging and refilling, and a longer cycle for carbon dioxide units, which are pressure vessels and are tested accordingly. What the monthly check catches is the thing that actually defeats an extinguisher, which is somebody moving it, hiding it behind boxes or using it and putting it back.
Other questions people ask
How often do fire extinguishers need to be inspected by staff?
Common practice is monthly, and the interval should be set in the fire risk assessment. A building where units are frequently moved or tampered with needs it more often, not less.
Can a member of staff do the annual service?
Only if they are genuinely competent to the relevant standard, with the equipment and spares to do it. In practice this is a specialist contractor in almost every building.
What happens if a service is missed?
The maintenance duty is continuing, so a lapsed service is a breach for as long as it lasts. The practical risk is worse: an unserviced unit that fails when used is the failure that gets investigated.
What should a fire extinguisher service label show?
The date of the service, what was done, the engineer and the next service due. It is the first thing an inspecting officer looks at, and a label with a date and no detail of the work is the usual finding on a cheap contract.