The Principal Act is amended -

(a) in section 20 by inserting the following after subsection (2):

"(2A) If at any time it appears for any reason an assessment is incorrect the Commissioners shall make such other assessment as they consider appropriate and any such assessment shall be substituted for the first-mentioned assessment.",

(b) in section 21(1) by substituting the following for the definition of "time for bringing an appeal":

"'time for bringing an appeal' means 30 days after the date of the assessment.",

(c) in section 21 by substituting the following for subsection (2):

"(2) An accountable person who is dissatisfied with an assessment of the Commissioners in relation to an instrument may appeal to the Appeal Commissioners against the assessment on giving, within the time for bringing an appeal, notice in writing to the Commissioners and the appeal shall be heard and determined by the Appeal Commissioners whose determination shall be final and conclusive unless the appeal is required to be reheard by a judge of the Circuit Court or a case is required to be stated in relation to it for the opinion of the High Court on a point of law.",

(d) in section 21 by substituting the following for subsection (3):