How much risk to do your sub-contractors present to you?

Very interesting talk at the SHE Show earlier this week regarding the court view of who is responsible when arrangements are put out to sub-contract.  A recent case was outlined where a council had outsourced management and operation of swimming pools to a contract organisation.  A child had tragically suffered brain damage following an accident in a swimming pool and whilst the initial finding was that the sub-contractor was responsible, a subsequent appeal resulted in a judgement that the council could not escape accountability.  The suggestion made by the QC who was presenting the case was that this case law is going to have major implications for the UK - how