IHTM30454 | Determination of questions on previous view of the law: payment and acceptance of tax
From HM Revenue & Customs · Inheritance Tax Manual
Apart from the other bars to assessment (), the general rule is that
where an assessment is in accordance with the law and practice at the time it is paid (and we have not expressly conceded or reserved the right to reopen) and
as a result of that payment tax has been paid in full
you should regard that payment as accepted in satisfaction of the tax liability for the purposes of at the earliest date on which we would have issued a clearance certificate () in respect of that liability.
You can find details of our practice in cases of delay by HMRC (), when tax is paid by instalments () and where there are amendments after the change of practice (). Subject to these situations, does not prevent you reopening any assessment where the tax assessed has not been paid in full.