For the purposes of certificates of tax deposit issued by the Treasury under section 12 of the [1968 c. 13.] National Loans Act 1968 on terms published before 31st July 1980, the date which is the due date in relation to—
(a)income tax charged at a rate other than the basic rate, and
(b)capital gains tax,
is by virtue of this section postponed, with respect to the year 1980-81 and any subsequent year of assessment, from the date specified in the prospectuses concerned to 1st December following the end of the year of assessment for which the tax is payable.