If you have any employees then you are obliged by law to provide suitable first aid facilities for them. A church is regarded as a low hazard environment and, provided there are fewer than 25 employees, there is no obligation to arrange formal training. There should, however, be at least one person who is responsible for looking after the first aid kit and checking and re-stocking it as necessary. Information about the requirements for a first aid kit can be found here.
While there is no specific legal obligation to provide first aid facilities for visitors or members of the congregation, it would be wise to assume that first aid is likely to be needed on occasion and have procedures in place
The Accident Book is an essential document for employers and employees to record and report details of specified work-related injuries and incidents.
You can purchase a book by following this link to the HSE website.
The HSE have produced guidance on various types of reportable incidents
The HSE have produced guidance on how to make a RIDDOR report.