12. For the purposes of Article 23 in the Schedule to the Act of 1965, as amended by section 4(2) of the Act of 1979, the value on a particular day of one special drawing right shall be treated as equal to such a sum in sterling as the International Monetary Fund have fixed as being the equivalent of one special drawing right:—
(a)for that day; or
(b)if no sum has been so fixed for that day, for the last day before that for which a sum has been so fixed.