“I think it important that there should be an effective means of detecting the kind of error which would entitle the court to intervene, and in practice I regard it as necessary for this purpose that the reasoning of the [decision maker] should be disclosed... It is to be noted that a principal justification for imposing the duty was seen as the need to reveal any such error as would entitle the court to intervene, and so to make effective the right to challenge the decision by judicial review.”