(1) Subject to subsection (2) and section 959AV, any additional tax due by reason of the amendment of an assessment for a chargeable period shall be deemed to be due and payable on the same day as the tax due under the assessment, before its amendment, was due and payable.
(2) Where -
(a) the assessment was made after the chargeable person had delivered a return containing a full and true disclosure of all material facts necessary for the making of the assessment, or
(b) the assessment had previously been amended following the delivery of the return containing such disclosure,
any additional tax due by reason of the amendment of the assessment shall be deemed to have been due and payable not later than one month from the date of the amendment.