Where a person is liable to a penalty under this Schedule, HMRC may assess the amount due by way of penalty and notify it to the person (subject to sub-paragraph (4)).

If it appears to HMRC that the amount that ought to have been assessed in an assessment under sub-paragraph (1) exceeds the amount that was assessed, HMRC may make a supplementary assessment of the amount of the excess and notify it to the person (subject to sub-paragraph (4)).

If it appears to HMRC that the amount that was assessed in an assessment under sub-paragraph (1) exceeds the amount that ought to have been assessed, HMRC may, by notice to the person, amend the assessment so as to reduce the amount due.

An assessment under sub-paragraph (1) or (2) may not be made after the end of the period of 2 years beginning with the time when facts sufficient in the opinion of HMRC to indicate that the person had failed as described in paragraph 4(1)(a) and (b) came to HMRC's knowledge.

An amendment under sub-paragraph (3) may be made after the last day on which the assessment in question could have been made.