I know, how does everyone get away with self employed lads, who only work for them !!
There is a questionnaire on hmrc.gov.uk that asks "is an employee self employed, or employed"
Usually if you are asking the question in the first place - they are employed, or should be!! In other words the govt wants Ni contributions out of you as well as their tax!!! It asks things like - do they have the right of substitution ie to replace labourer if they cant be present, do they have control over the hours they work, methods used, price charged etc etc in which case they are self employed but also the amount of work they do for you goes towards their actual employment status...
Another way round is to pay them on a commision only contract in which case you could seperate yourself from them easily.. but they would have to be set up with their own equipment as a sole trader filing their own tax return etc
Other way round is they have their own van and rounds, and sub, or they work for a few different lads... bit of a gray area really as it happens alot in practice, and is no doubt helped along by clever accounting, but id always think "what if they get wise to it" id hate to have to back pay NI!! Or be fined!