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.
Questions people ask about how often should a fire extinguisher be serviced
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.