They could not have a contract to carry out an illegal act
The statement sir is true only as far it goes.
One party may hire another to commit an illegal act. That would be a contract illegal at formation and
thus will not be enforceable in any court. Indeed it is cannot be a contract per se.
In this instance if the illegal act
is committed by one party on behalf of the other then it remains possible for both parties to be prosecuted for the offence depending on the circumstances.
Should one party enter into a legal contract with another where one party commits an illegal act in fulfilling the contract the courts may decide the contract is enforceable. It is most probable however that only the party who committed the illegal act would face prosecution.
The problem with PI's involved in the McCann case is we, the proles, do not know who were parties to the contract and what the contract was for.
We can have jolly good fun speculating though...