What is the test for fair comment in a defamation case?

This question has to do with the liberty which is to be allowed a defendant who has pleaded fair comment to adduce evidence on questions of fact. All comment must be based upon facts, and a newspaper article or a letter which comments upon a person may very well set out the facts which form the subject of the comment; but, whether it does so or not, the writer cannot justify his comment without establish ing’the truth of the facts upon which he relies. Comment, in order to be fair must be based upon facts, and if a defendant cannot shew that his comments contain no misstatements of fact, he cannot prove his defence of fair comment [per Lord Collins, M.R., in Digby v. Financial News, Ltd. (1907) 1 K.B. 502, at p. 507, 76 L.J.K.B. 321].

And in Lefroy v. Burnside (No. 2) 4 L.R. (Ir.) 556, the rule is laid down by Palles, C.B., as follows: * * the questions which would be raised at a trial by such a defence must necessarily be—first, the existence of a certain state ot facts, secondly whether the publication sought to be excused is a fair and bona Me comment upon such existing facts. If the facts as a comment upon which the publication is sought to be excused do not exist, the foundation of the plea fails.