SolvedCH157250 | Penalties for Failure to Pay on Time: Penalty assessments: supplementary assessment
From HM Revenue & Customs · Compliance Handbook
We can issue a supplementary assessment where
we have assessed a penalty and calculated it as a percentage of the tax unpaid, and
we later find that the amount unpaid was underestimated.
We can also amend an assessment where we find that the original assessment was excessive. In this situation the amendment does not affect when the penalty must be paid, and the amendment may be made after the last day on which an assessment could have been made, see CH157200.