F1SI 1985/454
48(1)The returning officer shall divide the number of valid ballot papers by a number exceeding by one the number of members to be elected.
(2)The result, increased by one, of the division under paragraph (1) (any fraction being disregarded) shall be the number of votes sufficient to secure the election of a candidate (in these rules referred to as the quota).
(3)At any stage of the count a candidate whose total vote equals or exceeds the quota shall be deemed to be elected, except that at an election where there is only one vacancy a candidate shall not be deemed to be elected until the procedure set out in paragraphs (1) to (3) of rule 51 has been complied with.