“The second method which could be adopted by the judge in these circumstances is himself to hear the evidence on one side and another, and come to his own conclusion, acting so to speak as his own jury on the issue it which is the root of the problem. The third possibility in these circumstances is for him to hear no evidence but to listen to the submissions of counsel and then come to a conclusion. But if he does that, then…where there is a substantial conflict between the two sides, he must come down on the side of the defendant.”