What is the test for fair comment?

In Silkin v. Beaverbrook Newspapers Ltd., [1958] 1 W.L.R. 743, Diplock, J. in charging the jury explained at p. 747 the test for fair comment in the following passage which has become a formulation of the test widely accepted in the authorities and sometimes described as the "cardinal test": But the expression "fair comment" is a little misleading. It may give you the impression that you, the jury, have to decide whether you agree with the comment, whether you think it is fair ... applying the true test: was this an opinion, however exaggerated, obstinate or prejudiced which was honestly held by the writer?