What is the test for defamatory statements to be made in a libel action?

As Lord Diplock stated in Horrocks v. Lowe, [1974] 1 All E.R. 662 (at p. 669, para. c): “So, the motive with which the defendant on a privileged occasion made a statement defamatory of the plaintiff becomes crucial. The protection might, however, be illusory if the onus lay on him to prove that he was actuated solely by a sense of the relevant duty or a desire to protect the relevant interest. So he is entitled to be protected by the privilege unless some other dominant and improper motive on his part is proved. ‘Express malice’ is the term of art descriptive of such a motive. Broadly speaking, it means malice in the popular sense of a desire to injure the person who is defamed and this is generally the motive which the plaintiff sets out to prove. But to destroy the privilege the desire to injure must be the dominant motive for the defamatory publication; knowledge that it will have that effect is not enough if the defendant is nevertheless acting in accordance with a sense of duty or in bona fide protection of his own legitimate interests.”