3All sums paid or payable by way of excess profits tax or the national defence contribution for any relevant chargeable accounting period in respect of any trade or business carried on by any member of a group of companies shall, for the purpose of ascertaining whether any, and if so what, post-war refund is due to any person, be deemed to have been paid or to be payable by the principal company of the group and not by any other member thereof ; and for the purposes of Part IV of this Act, the trade or business of the principal company shall be deemed to be the original trade or business.