Section 3 of the Health and Safety at Work etc. Act 1974 makes every employer responsible for people who are not its employees but are affected by its undertaking, which includes the contractors it brings in. The Management Regulations add the duties to cooperate and coordinate where two employers share a workplace and to give a visiting contractor the information it needs. CDM 2015 puts it more sharply for construction: regulation 8 requires anyone appointing a designer or contractor to check it has the skills, knowledge, experience and organisational capability for the work, and regulation 15 sets out what every contractor must then do.
The HSE’s guidance, Using contractors: a brief guide, sets out the cycle that contractor management follows: plan the work and decide what the contractor needs to know, choose a contractor who is competent for it, brief and induct them, cooperate and coordinate while the work is done, monitor it, and review both the work and the contractor at the end. Pre-qualification through an SSIP scheme such as CHAS, SafeContractor or Constructionline answers the paperwork question; it does not answer whether the gang who turn up on Monday can do the job safely.
On a live site the discipline is continuous. Subcontractor RAMS are reviewed before the work starts and again when the work changes; supervisors are named; permits control the tasks where sequence matters; inspections check what is actually happening; and a subcontractor that cannot meet the standard is supported to reach it or stopped.