HMRC considers Covid-19 as a reasonable excuse for failure to comply with certain tax obligations (such as payments or filing dates), provided failure to comply is remedied as soon as it is possible to do so.
In cases other than Covid-19 related, HMRC does not require supporting evidence to be submitted with any claim but may later ask for evidence. However, if you appeal a penalty on the grounds of Covid-19, you will be required to explain how you were affected if claiming 'reasonable excuse'.
HMRC also extended the normal 30-day deadline for appeals by an additional three months for Covid-19 related failures. This extension applies to penalties issued in February 2020 or later.