15On a claim being made in that behalf by the appropriate person there shall be determined for the purposes of this Part of this Schedule the amount (if any) by which—
(a)the aggregate of the values which, apart from this Part of this Schedule, would be the values for the purposes of tax of all the qualifying investments comprised in a person's estate immediately before his death which are sold by the appropriate person within the period of twelve months immediately following the date of the death
exceeds
(b)the aggregate of the values of those investments at the time they were so sold, taking the value of any particular investments for this purpose as the price for which they were so sold or, if it is greater, the best consideration which could reasonably have been obtained for them at the time of the sale.