You must establish who is responsible for what, and it is recommended that you should always have written evidence of what has been agreed. It is essential that you check the adequacy of third party contractors' public liability insurance arrangements. You should establish, in writing (preferably from their insurer or their insurance broker), that it provides at least a £5m limit of indemnity and will be valid for the whole of the contract period. It is equally important that it does not carry any exclusions that may be relevant, e.g. one that excludes the use of a naked flame if the contract that you are entering into includes "hot work activity" (re-covering a flat roof, for example).