As is said above you need Employers Liability Insurance, which is a requirement by law irrespective of whether they work directly for you or work on a casual basis.
Getting them to sign something that they can't sue you for negligence isn't worth the paper it is written on, as mentioned above a court will just ignore it. You can get them to sign a waiver with regards to payment though, (for instance that a waiver that they cant sue you for interest on late payments, or that they can't sue you for their wages outright is another one, that you'd go the mediation route or something like that) ... but no labourer in their right mind would sign that.
But as for a waiver for negligence and personal injury there is nothing a court will take note of.