IHTM04345 | Estate Duty surviving spouse exemption: procedures where relief is due
From HM Revenue & Customs · Inheritance Tax Manual
If an interest in settled property (IHTM16000) is included on Schedule IHT418 or form IHT100 and
the interest arose under the Will or intestacy of the deceased’s spouse,
the spouse died before 13 November 1974, and
the taxpayer has deducted relief in full under IHTA84/SCH6/PARA2
you can accept that the property may be left out of account without further investigation.