13.—(1) In this Part, in relation to each class of hereditaments–
(a)in any formula–
T is the amount specified in relation to that class in column 1 of the Schedule;
y is the relevant amount of water in relation to the period of three years ending immediately before the relevant preceding year;
Y, subject to article 14(3), is the relevant amount of water in relation to the period of three years ending on 31st March 1989;
(b)“relevant amount of water” in relation to a relevant undertaker and any period of three years means the amount calculated in accordance with paragraph (2); and
in relation to a class of hereditaments “relevant undertaker” means the water undertaker in occupation of those hereditaments.
(2) The relevant amount of water in relation to a relevant undertaker and any year is the annual average, over the three years being considered, expressed in megalitres to the nearest tenth part of a megalitre, of the amount estimated in accordance with sub-paragraph (a) less the amount estimated in accordance with sub-paragraph (b); that is to say–
(a)the estimated annual average amount of water put into supply by that undertaker in the period of three years, being water supplied both–
(i)to any other person in bulk, that is to say for the purpose of augmenting or constituting the supply to be given by that person and
(ii)otherwise,
less half the estimated annual average amount of non-potable water so supplied otherwise than in bulk; less
(b)half the annual average amount estimated to have been taken or supplied in bulk during that period of three years.
(3) Where the relevant undertaker did not supply water throughout any year in a period of three years, paragraph (2) shall apply as if for the reference to the annual average over three years there were substituted a reference to the number of years throughout which water was supplied; and for the purposes of this paragraph any temporary suspension of supply shall be disregarded.