TCTM10710 | Time limits for penalties
From HM Revenue & Customs · Tax Credits Technical Manual
Paragraph 6 of Schedule 2 of the Tax Credits Act 2002 provides time limits for penalties. The time limit for penalties is the later of:-
one year after the end of the period during which you could have made an enquiry (the date the enquiry window would have closed) and
the end of the period of one year beginning with the day on which the S19 enquiry is completed.
If the time limit for penalties has expired a penalty determination can no longer be considered nor can a penalty be included in a letter of offer after that date.