Other income due to the deceased: Rents due to the deceased
You should check the other guidance available on GOV.UK from HMRC as Brexit updates to those pages are being prioritised before manuals.
Box 76 of the IHT400 should include the gross amount of any rent from let property that was due to the deceased, but had not been paid, at the date of death.
You can normally accept the figure given without enquiry unless you have clear evidence to suggest that there has been an undervaluation or omission. But you should check that property that produces rent has been included on form IHT405. Rents due and properly payable in advance should be included at full value.