What is the legal test for assessing a defendant’s response to attack?

Another leading authority on the issue is Horrocks v. Lowe, [1975] A.C. 135 (HL) [Horrocks], which has been cited with approval in our courts on numerous occasions. The generous approach to be taken in assessing a defendant’s “response to attack” was described by Lord Diplock at 151 as follows: The exception [to the protection of qualified privilege] is where what is published incorporates defamatory matter that is not really necessary to fulfilment of the particular duty or the protection of the particular interest upon which the privilege is founded. Logically it may be said that such irrelevant matter falls outside the privilege altogether. But if this were so it would involve the application by the court of an objective test of relevance to every part of the defamatory matter published on the privileged occasion; whereas, as everyone knows, ordinary human beings vary in their ability to distinguish that which is logically relevant from that which is not and few, apart from lawyers, have had any training which qualifies them so to do. So the protection afforded by the privilege would be illusory if it were lost in respect of any defamatory matter which upon logical analysis could be shown to be irrelevant to the fulfilment of the duty or the protection of the right upon which the privilege was founded.