2For the purposes of this Part of this Schedule, a group of companies shall be deemed to be the same group notwithstanding any changes in the members thereof so long as, and only so long as, the same body corporate remains the principal company of the group ; and references in this Part of this Schedule, in relation to a member of a group of companies, to relevant chargeable accounting periods shall be construed as not including references to any chargeable accounting period during which it was not a member of the group.