There is a very (understandable) common misconception within the building / tradesman industry with regards to who is classed as an employee in the eyes of the insurers and the law.
There are 4 types of people who could be appointed by a main contractor and I will explain your obligation toward them if you are the main contractor:
Labour only Subcontractors
These are people directly employed by the main contractor who work for them on a permanent basis. Obviously the main contractor is obliged by law to provide Employers Liability cover for these people.
Labour only Subcontractors (LOSC):
This is one where most contractors get it wrong. These are people who are subcontracted in by the main contractor (as an extra pair of hands) to help the main contractor with the day to day tasks in the contract. These people will usually be working under the main contractors instruction and will be using the main contractors materials. Regardless of if the LOSC is self employed themselves or not and has their own Public Liability policy, the main contractor will still need to cover them as an employee.
To give an example:
A self employed window fitter (John) wins a job to install a large skylight. The nature of the job requires him to contract in his friend (Dave, who is also a self employed window fitter) to help with the installation. Dave will be working under Johns instruction and will also be using Johns materials. As such John will need employers liability to cover Dave in the event that he instructs him to do something that causes him injury.
These are people who are contracted in to perform a task that the main contractor wouldn't do themselves. They will not work under any instruction from the main contractor and will use their own materials.
E.g A builder is building an extension on their clients home, he may subcontract work to an electrician to wire the extension up as the builder does not do electrical work themselves. The electrician will price and invoice the job to the main contractor as though the main contractor was a normal client, will not work under any instruction and will use their own materials.
Bona-fide Subcontractors do not need to be covered as employees. It is the main contractors responsibility to ensure that any BFSC they use has their own Public Liability policy in force though.
Volunteers / Work experience
People appointed by the main contractor doing unpaid voluntary work or work experience need to be covered as employees.
These are a general guideline and are under the assumption that the main contractor appoints and pays the 4 categories above. It can be a bit more complicated if the client starts appointing multiple tradesmen themselves.
In any case it is always best to speak to your insurance advisor to ensure you are adequately covered.
For more information on what Advance Insurance do, please see our website www.advanceinsurance.co.uk