IHTM30321 | End of instalment option: introduction
From HM Revenue & Customs · Inheritance Tax Manual
The persons paying the tax by instalments may change their minds and pay the outstanding balance of the tax in one sum with accrued interest whenever they wish.
The outstanding balance of the tax becomes payable in full immediately if
But if the above events take place before the date when the first instalment would otherwise be due, the full tax and interest becomes payable on the first instalment date.
Where only part of the qualifying property is sold, a proportionate part of the tax and interest becomes payable.
There are additional special provisions where the chargeable transfer was made otherwise than on death (IHTM30324).