There is usually a 5% surcharge added to any Self Assessment tax outstanding at the end of February. For 2021, this has been delayed and will only be levied at the end of March. Another small but useful concession at a time when cashflow is a significant problem for so many.
Where a ‘time to pay’ arrangement has been put in place, in most cases no surcharge will apply, although interest does still run on any amount overdue (currently at 2.6%).