The simple answer is that both the Prison Service and the Agency have a duty of care to ensure the health and safety of the prisoner SFARP. The prisoner even though is out for day release will always remain in the custody of the prison as they will be under licensed conditions which is issued and signed by the governor of the prison. The Prison Service have a legal obligation to ensure the safety of the prisoner at all times, even when the prisoner is placed on day release under the management of the agency.

The Prison Service has a due diligence to check the competency of specialist agency to ensure and satisfy themselves that the industries where the prisoners are placed with conform to all the health & safety policies, risk assessments, safe systems of work and requirements to prevent ill health or an injuiry to the prisoner SFARP 

The agency also have a duty of care under the HASAWA to ensure that prisoners are fully inducted and made aware of the risks, policies & procedures and what control measures they need to take to reduce the likelyhood of them having an accident / incident. The agency should also make due diligemnt H&S checks of where prisoners are going to be placed to ensure that the industries are competent and meet the required H&S standards. 

I hope this helps!

All the best