Interest on delayed excise duty accrues from three months after determination at Board fixed rates until payment is made. A new provision requires payment of interest on excise duty determined under section 11A if unpaid within three months, at a Board fixed rate within a prescribed band, accruing from the day after the three month period until payment; a transitional proviso covers determinations made before assent. Explanations specify that where appeals or court orders reduce, increase, or further increase duty, the operative date for interest is the date on which each respective amount was first determined payable.