Ian (Griffiths) is correct.
If they work as a Bona Fide Sub Contractor and they invoice you, use their own equipment then generally they will not need to be covered by your policy for Employers Liability. You may still wish to extend your public liability policy to cover "contingent liability" in the event that they fail to tell you their cover has expired and a claim is made against your policy. However, it is a condition of most insurance companies that you check annually that such BFSC's have cover in place so these instances are not very common.
If they work as self employed but under your guidance and you pay them whether it be hourly, weekly or whatver then they are, to all intents and purposes an employee and need to be covered by Employers Liability Insurance. However, we are only here to advise, not enforce the law, but the HSE can do that.