(5) Insurance periods under paragraphs (4) and (8) shall be treated as follows:
(a)as if they had been completed under the legislation of any part of the United Kingdom under which a pension is, or if such periods are taken into account, would be, payable; or
(b)where a pension is, or would be, payable under the legislation of two or more parts of the United Kingdom as if they had been completed under the legislation of that part which, at the date on which entitlement first arose or arises, is paying, or would pay, the greater, or greatest, amount.