Paragraph 13 I1

an assessment has been notified to a person under paragraph 12, and

it appears to the Commissioners that the amount which ought to have been assessed as due exceeds the amount that has already been assessed.

on or before the last day on which the assessment under paragraph 12 could have been made, make a supplementary assessment of the amount of soft drinks industry levy due from the person, and

notify the amount to that person.