The HMRC inheritance tax section is part of Trusts & Estates, so the address is:

HMRC Trusts & Estates
Ferrers House
PO Box 38
Castle Meadow Road
Nottingham
NG2 1BB

(Some of us still remember it as the “Capital Taxes Office”.)

Update (2021): the new (and very short) addresss refers to Inheritance Tax again.

Inheritance Tax
HM Revenue and Customs
BX9 1HT

But the “BX” postcode doesn’t allow recorded delivery on the Royal Mail system, as it is a dummy postcode for post scanning purposes, although couriers may use this:

HM Revenue and Customs
Benton Park View
Newcastle Upon Tyne
NE98 1ZZ