Defamation law exists to protect the reputation of a person (or company) from defamatory statements made about him/her to a third party without lawful justification. A statement is defamatory if, when said about a person and published to a third party, it would make reasonable people think less of that person. For a person to sue they must show;
a) that the defamatory language was used about them
b) that they were identified or identifiable and
c) that the words were published to another, i.e. a third party.
It doesn't matter whether the statement is intended to be defamatory. The judge trying the case will decide what the broadcast is saying and whether it has unjustifiably injured the claimant’s reputation. Since the introduction of the Defamation Act 2013 which came into force on 1 January 2014, for a claimant to sue over a defamatory statement, the claimant must show that publication caused or is likely to cause serious harm. Bodies trading for profit e.g. companies, must show that publication caused or is likely to cause serious financial loss. This new threshold is intended to raise the bar and to prevent trivial claims.
See 'What is Defamatory?'.