It is a defence to the publication of defamatory matter if the defendant proves that--
(a) the matter carried, in addition to the defamatory imputations of which the plaintiff complains, one or more other imputations (
"contextual imputations" ) that are substantially true, and
(b) the defamatory imputations do not further harm the reputation of the plaintiff because of the substantial truth of the contextual imputations.