The answer is not "one per fifty staff". It is also not whatever you had last year. The law expects you to work the number out yourself, and to be able to show how you got there.
What the law asks
The Health and Safety (First-Aid) Regulations 1981 require employers to provide "adequate and appropriate" first aid equipment, facilities and people. The regulations do not set a number. The HSE's guidance, L74, explains how to decide, and it starts with a needs assessment.
Six questions that set the number
1. How hazardous is the work? An office is not a quarry. Plant, vehicles, chemicals, work at height and machinery all push the number up.
2. How many people, and where? Count every site, floor and building. A first aider in head office does nothing for a driver in Carlisle.
3. When do people work? First aid cover has to be there on every shift, including nights and weekends, not just from nine to five.
4. Who covers holidays and sickness? If your only first aider is on leave, so is your cover. Plan for absences.
5. Who works alone or remotely? Lone workers and drivers need their own arrangements: kits, training and a way of raising the alarm.
6. What has actually happened? Your accident book tells you what injuries are likely, which in turn tells you what level of training you need.
A starting point, not an answer
L74 gives suggested starting figures. In lower-hazard workplaces, it suggests an appointed person for fewer than 25 people, at least one Emergency First Aid at Work trained person for 25 to 50, and one First Aid at Work trained person for every 100 people above that. In higher-hazard workplaces, the figures are tighter: an appointed person for fewer than 5, at least one trained first aider for 5 to 50, and one more First Aid at Work trained person for every 50 people above that.
Treat those figures as the floor and adjust them using the six questions above.
The bits that get missed
Certificates expire after three years, and HSE strongly recommends an annual refresher in between. Visitors and the public are not legally covered by the regulations, but HSE strongly recommends including them in your assessment. Mental health is now part of the conversation too. Your assessment should consider whether people trained in mental health first aid would help your workforce.
Where PMSCS helps
PMSCS can carry out the needs assessment with you, record it properly and deliver the training it identifies. That includes regulated FAA First Aid at Work, Emergency First Aid at Work and Mental Health First Aid courses, delivered through NUCO.
This article is general guidance, not legal advice.