No appeal may be made against the amount of any income, profits or gains or, as the case may be, chargeable gains, or the amount of any allowance, deduction, relief or tax credit specified in an assessment or an amended assessment made on a person for a chargeable period where either -
(a) a Revenue officer has determined the amount by accepting without alteration of and without departing from the statement or statements, or the particular or particulars with regard to income, profits or gains or, as the case may be, chargeable gains, or allowances, deductions, reliefs or tax credits specified in the return delivered by the person for the chargeable period, or
(b) the amount has been agreed between the Revenue officer and the person, or any person authorised by the person in that behalf, before the making of the assessment or the amendment of the assessment, as the case may be.