Reviews and appeals for direct taxes: Settlement of appeals: Finality of settlement
Once the appeal has been settled, whether by a tribunal decision or by agreement with HMRC, it is final and conclusive unless there is legal power to revisit it.
For example tribunal decisions may be appealable to the Upper Tribunal or relevant appellate court. There is no provision to reopen an agreement under S54 TMA 1970 or equivalent (except within the 30-day time limit for customers to change their mind after the agreement).
The decision maker should note that settlement under S54 TMA 1970 (or equivalent) has the same effect as if the appeal was decided by the tribunal, and take care when drafting any agreement letter.