Who HMRC are contacting
From April 2022, HMRC will be writing to taxpayers whose entitlement to the fourth and/or fifth SEISS grant has gone down by more than £100, asking them to repay the overpaid amount. HMRC will make it clear how they have worked this out and the steps taxpayers need to take.
If an amendment to their tax return has been made after 3 March 2021 for any of the years 2016-17 to 2019-20, which means they are no longer entitled to the amount they originally claimed for their fourth and/or fifth SEISS grant, they need to repay the amount they were overpaid.
If this has happened, HMRC will also look at their turnover figures if they claimed the fifth SEISS grant. If they are now eligible for the lower (30%) grant, but claimed the higher (80%) grant, HMRC will also ask them to repay the difference.
See more at https://www.tax.org.uk/overpayments-of-seiss-grants-hmrc-update