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HMRC internal manual

Inheritance Tax Manual

Assessing: calculating interest: decrease in total IOP (WIR) tax

Where there is a decrease in the total tax, you can easily include the adjustment in the calculation for the next instalment (IHTM30191).

Remember to repay any interest that was overpaid on the tax that is now repayable (IHTM31616). You should calculate this by looking at the interest charged on each instalment in turn.