IHTM31611 | Assessing: repayments: summary
From HM Revenue & Customs · Inheritance Tax Manual
You should calculate the tax to pay at the assessment group in the usual way (IHTM31401). Where this results in an overpayment the calculation will show an amount to repay. As well as the overpaid tax, you must also repay any overpaid interest in respect of that tax. Consider both the tax and interest in deciding whether you must make the repayment (IHTM31601).
The general principle is that you must repay
the amount of tax attributable to the overpayment of tax
on a ‘last in - first out’ basis.