21.—(1) No prosecution for an offence under this Part of these Regulations which is punishable under regulation 20(1) shall be begun after the expiry of—
(a)3 years from the commission of the offence; or
(b)1 year from the date on which evidence sufficient in the opinion of the prosecutor to warrant the proceedings came to the knowledge of the prosecutor,
whichever is the earlier.
(2) For the purposes of this regulation, a certificate signed by or on behalf of the prosecutor and stating the date on which evidence sufficient in the prosecutor’s opinion to warrant the proceedings came to the knowledge of the prosecutor, shall be conclusive evidence of that fact.
(3) A certificate stating that matter and purporting to be so signed shall be deemed to be so signed unless the contrary is proved.