6. An employer who is required by a written notice issued by an inspector to do so shall produce or send to any person specified in the notice, at the address and within the time specified in the notice—
(a)either the original or a copy of every certificate issued to him under regulation 4 which relates to a period of insurance current at the date of issue of the notice;
(b)either the original or a copy of every certificate issued to him under regulation 4 and retained by him in accordance with regulation 4(4).