F1SI 1985/454
17(1)Every ballot paper shall be marked with an official mark, which shall perforate or emboss the ballot paper.
(2)The official mark shall be kept secret, and an interval of not less than seven years shall intervene between the use of the same official mark at elections for the same district electoral area.
(3)The official mark used for ballot papers issued for the purpose of voting by post shall not be used at the same election for ballot papers issued for the purpose of voting in person.