Supplementary assessments
8If it appears to the Commissioners that the amount which ought to have been assessed in an assessment under paragraph 3, 4 or 7 above exceeds the amount which was so assessed, then—
(a)under the same paragraph 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 notify the person concerned accordingly.