Andrew Try looking at Regulations 11 and 12 of the Management at Work Regulations 1999, that should give you a start. Unfortunately the ACoP has been withdrawn as part of the "improvement" process.

Tell subcontractor 1 that he is fully responsible for 2 and 3 as if they were his own employees. Contractually he is responsible and I am confident the HSE would also see it this way.

Assuming this is not a transient occurrence (ie short construction) but a long-standing involvement of several parties in the process, I would expect there to be some contractual arrangements in place which should clearly spell out the responsibilities and duties, including any sub-contract works. That would be my first point of call for clarification. Regardless of what H&S law may have to say about this, I would also want to be sure about performance: if Contractor 3 or 2 fails to deliver their promised product or service, how will that effect the overall situation? Complete failure, delay, unfit product, financial effect? These may be just as significant as the safety and health implications.

Ray Spoken like a true construction person:-)

Supply chain management controls 'should' be built into the first contract. If they are not - expect chaos and confusion. In a previous position, I was flown half way round the world by an oil major to investigate an accident... They had superb direct contractor controls but had completely neglected to specify supply chain checks, and had no effective control whatsoever on who was employed on site.