(1) If within the time prescribed by section 19(3) an accountable person fails to furnish in accordance with the relevant regulations a return of the tax payable by him in respect of any period, then, without prejudice to any other action which may be taken, the Revenue Commissioners may, in accordance with regulations, but subject to section 30, estimate the amount of tax payable by him in respect of that period and serve notice on him of the amount estimated.
Provided that where the Revenue Commissioners are satisfied that -
(a) the amount so estimated is excessive, they may amend the amount so estimated by reducing it, or
(b) the amount so estimated is insufficient, they may amend the amount so estimated by increasing it, then, in either case, they shall serve notice on the person concerned of the revised amount estimated and such notice shall supercede any previous notice issued under this subsection.
(2) Where a notice is served under subsection (1) on a person, the followin