Defence of publication in the course of business
21. In proceedings for an offence under any of the preceding provisions of this Part consisting of the advertisement for sale of any food, it shall be a defence for the person charged to prove—
(a)that he is a person whose business it is to publish or arrange for the publication of advertisements; and
(b)that he received the advertisement in the ordinary course of business and did not know and had no reason to suspect that its publication would amount to an offence under that provision.