What is the test for a "fair comment"?

The word "fair" in the phrase "fair comment" is a misnomer because it conveys the concept that comment must be "reasonable". This is not the case as pointed out by Diplock J. in addressing the jury in Silkin v. Beaverbrook Newspapers, [1958] 1 W.L.R. 743, [1958] 2 All E.R. 516 at 520, when he said: "So in considering this case, members of the jury, do not apply the test of whether you agree with it. If juries did that, freedom of speech, the right of the crank to say what he likes, would go. Would a fair-minded man holding strong views, obstinate views, prejudiced views, have been capable of making this comment? If the answer to that is yes, then your verdict in this case should be a verdict for the defendants. Such a verdict does not mean that you agree with the comment. All it means is that you think that a man might honestly hold those views on those facts."