You may be aware that 2020 tax returns with an original filing date of 31/01/21 were delayed until 28/02/2021 with no penalty for the late filing of the return.
This is correct, however the tax that is due 31/01/2021 remains due at that date. This is a double whammy for a lot of self employed as well as the normal payments due at this time the delayed installment from 31/07/2020 was also due 31/01/2021.
As a consequence interest and late payment penalties are accruing.
If you are unable to make payment now you may be able to apply for time to pay. In some instances you can apply online at www.HMRC.gov.uk or call the self assessment help line and apply that way.
If payment by installments is accepted then Interest will continue to accrue.
I'm a Accountant that has dealt with many SME's ( Small and Medium sized Enterprises) over the years, in all aspects of Accounting, Business advice etc.