“It seems to me that the law has now been developed to the point where, unless the source of power clearly provides the answer, the question whether the decision of a body is amenable judicial review requires a careful consideration of the nature of the power and function that has been exercised to see whether the decision has a sufficient public law element, flavour or character to bring it within the purview of public law. It may be said with some justification that this criterion for amenability is very broad, not to say question-begging. But it provides the framework for the investigation that has to be conducted.”