21. Section 81(1) of the Act shall not apply where the grounds on which the person concerned is aggrieved are that—
(a)any assumption as to the future that is required by Part V of the Administration Regulations to be made in the calculation of an amount payable as council tax or council water charge may prove to be inaccurate; or
(b)the calculation of an amount payable as council tax fails to take proper account of the provisions of the Council Tax Benefit (General) Regulations 1992(1).