Under the Health and Safety at Work etc. Act 1974 (HASAW) primary responsibility for health and safety lies with the employer. However, all employers make use of from time to time of contractors/sub contractors, and it is incumbent on employers to ensure that those contractors engaged, are indeed fit for purpose with regards to the intended task and/or undertaking.
Whilst employers have a duty to inform and consult with contractors on all aspects, including health and safety, contractors themselves cannot dissolve their responsibility back to their "employer".
Under Section 2 of the HASAW Act, they must take all reasonable and practicable steps which shall be safe and without risk to other persons.
It is therefore a good idea to have a list of "Approved Contractors" which has been established against given criteria; typically this will include health and safety arrangements/ performance, capabilities and limitations.
Under the Construction (Design Management) Regulations (CDM), contractors (Principal or otherwise) have specific duties to undertake with regards to co-operation with the client and Planning Supervisor, in the preparation of Risk Assessments, Method Statements and health and safety plans. Likewise if the contractor engages sub contractors, there is a similar passage of duty and responsibility.