If tax has been underpaid, the balance is payable with interest, unless either:
•a certificate of discharge (see ¶183-365) has been issued; or
•the tax already paid has been accepted by the Board in full satisfaction of the tax liability, and more than six years have elapsed from the date of payment (the last payment in the case of instalments): IHTA 1984, s. 240(1) and (2).
In any case of fraud, wilful default or neglect by the person liable, the period of six years begins at the time the fraud etc. comes to the knowledge of the Board (s. 240(3)).