20.—(1) Proceedings for an offence under these Regulations may be commenced within the period of one year from the date on which evidence sufficient in the opinion of the prosecutor to warrant the proceedings came to his knowledge.
(2) But no such proceedings shall be commenced by virtue of paragraph (1) more than three years after the commission of the offence.
(3) For the purposes of this regulation—
(a)a certificate signed by or on behalf of the prosecutor and stating the date on which evidence sufficient in his opinion to warrant the proceedings came to his knowledge shall be conclusive evidence of that fact; and
(b)a certificate stating that matter and purporting to be so signed shall be deemed to be so signed unless the contrary is proved.