If you only give them the odd job or two and they are a full time business then the only requirement you need to do is verify their H&S Policies, Saftey Policy, Insurance Certificate and Risk Assessment.
If an accident was to occur, you as the hirer are liable to have checked all this information before awarding the contract. If you do not have copies of the above along with your own documentation confirming the required checks and documents have been verified then you can also be held liable for prosecution
Otherwise, they should not be covered under your employers liability insurance as they are bonafida sub contractors.
Some Insurance companies will make a provision on your insurance to cover them, but guess who pays for that!!