(a)the registrable person and an authorised person have agreed in writing that weight shall be determined in accordance with rules other than those described in regulation 42 or specified under regulation 43; and
(b)a direction under paragraph (3) below has not been made.
44(2) rules may be agreed under this regulation as regards–
(a)the method by which weight is to be determined;
(b)the time by reference to which weight is to be determined;
(c)the discounting of water forming a constituent of material disposed of, but this is subject to paragraph (5) below.