17The polls at an election of councillors for any ward of a London borough and at an election of a councillor of the Greater London Council for any electoral area which includes the whole or part of that ward shall, so far as they relate to polling districts which are the same for both elections, be taken together—
(a)in the case of all ordinary elections held in or after the relevant year of election ; and
(b)in the case of elections to fill casual vacancies in both offices where the same day of election is fixed for both elections.