12Certificates of tax deposit issued by the Treasury under section 12 of the [1968 c. 13.] National Loans Act 1968 on terms published on or before 14th May 1979 may be used for making payments of duty; and for that purpose those terms shall have effect with the necessary modifications and as if the duty in or towards the payment of which a certificate is used were due—
(a)in the case of duty payable under paragraph 9 or 10 above, two months after the end of the chargeable period to which it relates ;
(b)in the case of duty payable under paragraph 11 above, at the end of the month in which it is required to be paid.