After notification of an assessment has been given to a person under section 211(2), the assessment may not be altered except in accordance with this section or on appeal.

A supplementary assessment may be made in respect of a penalty if an earlier assessment operated by reference to an underestimate of the value of the denied advantage.

An assessment or supplementary assessment may be revised as necessary if it operated by reference to an overestimate of the denied advantage; and, where more than the resulting assessed penalty has already been paid by the person to HMRC, the excess must be repaid.