What is the test for express malice in the context of privileged privilege?

In Horrocks v. Lowe, cited above, Lord Diplock said this at p. 670 All E.R.: “…Qualified privilege would be illusory, and the public interest that it is meant to serve defeated, if the protection which it affords were lost merely because a person, although acting in compliance with a duty wearing protection of a legitimate interest, disliked the person whom he defamed or was indignant at what he believed to be that person’s conduct and welcomed the opportunity of exposing it. It is only where his desire to comply with the relevant duty or to protect the relevant interest plays no significant part in his motives for publishing what he believes to be true that “express” malice can properly be found.” (my emphasis) Although those remarks were directed more to the question of malice, they are in my view equally applicable to the question of whether the limits of the privileged occasion have been exceeded.