20.—(1) Proceedings for an offence under regulation 18 above may, subject to paragraph (2) below, be commenced within a period of six months from the date on which evidence sufficient in the opinion of the prosecutor to warrant proceedings comes to the prosecutor’s knowledge.
(2) No such proceedings shall be commenced by virtue of this regulation more than twelve months after the commission of the offence.
(3) For the purpose of this regulation a certificate signed by or on behalf of the prosecutor stating the date on which evidence sufficient in the opinion of the prosecutor to warrant the proceedings came to the knowledge of the prosecutor shall be conclusive evidence of that fact.
(4) A certificate stating that fact and purporting to be so signed shall be deemed to be so signed unless the contrary is proved.