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HMRC internal manual

Inheritance Tax Manual

Investigating the assets on page7 - Income Tax or Capital Gains Tax repayment (box 74)

Box 74 of the IHT400 should include any Income Tax or Capital Gains Tax actually repaid to the estate or a reasonable estimate of any repayment that might be due to the deceased.

You can generally accept the value offered, including estimates, unless you have evidence to suggest that the value might be understated.