For the purposes of the provisions of this Act relating to the vacation of the seat of a member of the House of Commons who becomes disqualified by this Act for membership of that House, the office of steward or bailiff of Her Majesty's three Chiltern Hundreds of Stoke, Desborough and Burnham, or of the Manor of Northstead, shall be treated as included among the offices described in Part III of Schedule 1 to this Act.