RM2588 | Issuing manual payable order: repayment supplement: introduction
From HM Revenue & Customs · Repayment Claims Manual
When supplement is due it is calculated at a rate of interest on the amount of the delayed repayment. The rate of interest is applied for a period starting at what is called therelevant time' and ending at the end of the tax month’ in which either the repayment or set off is made.
The computer will calculate supplement for you except when
a previous manual payable order has been issued
or
there have already been four or more repayment computations for the year
or
you entered N in REPAYMENT SUPPLEMENT DUE
In the above circumstances you have to calculate any supplement manually. To do this decide