Supplementary assessments
27If, otherwise than in circumstances falling within subsection (5)(b) of section 56 of this Act, it appears to the Commissioners that the amount which ought to have been assessed in an assessment under any provision of that section or under paragraph 25 above exceeds the amount which was so assessed, then—
(a)under the like provision as that assessment was made, and
(b)on or before the last day on which that assessment could have been made,
the Commissioners may make a supplementary assessment of the amount of the excess and shall notify the person concerned accordingly.