3(1)Where duty under section 115 has been paid—
(a)on the original sale, or
(b)on an intervening subsale,
duty under that section on a subsale, or subsequent subsale, is chargeable only in respect of the amount (if any) by which the chargeable consideration on that transaction exceeds the chargeable consideration on the earlier transaction.
(2)If there is more than one such earlier transaction on which duty has been paid, the reference in sub-paragraph (1) to the chargeable consideration on the earlier transaction shall be read as a reference to the higher or highest amount of chargeable consideration on which duty has been paid.
(3)If the subsale does not relate to the whole of the property to which the earlier transaction related, the references in sub-paragraphs (1) and (2) to the chargeable consideration on an earlier transaction shall be read as references to an appropriate proportion of that consideration.
(4)What is an appropriate proportion shall be determined on a just and reasonable basis having regard to the subject matter of the subsale and of the earlier transaction.
(5)For the purposes of this paragraph the chargeable consideration on a transaction is the consideration that falls to be brought into account in determining the duty chargeable on it.
(6)Where under this paragraph duty on a subsale is chargeable in respect of part only of the consideration for the subsale, it is chargeable at the rate that would be applicable if the whole of the chargeable consideration on the subsale were taken into account.