(1) Subject to this section, in relation to an assessment made on any person for any year of income the Comptroller is satisfied upon a claim made within 6 years after the end of that year of income that there is a mistake in the assessment apparent from the face of the return, the assessment or other records, as a result of which—
(a) the tax charged is greater than the amount which should be charged;
(b) any assessed loss is less than the amount at which it should be assessed; or
(c) a refund is made which is less than the amount which should have been refunded, the Comptroller shall make a reduced assessment accordingly.
(2) Where, on the determination of an appeal made under Part 11, the appeal commissioners or any subsequent appellate tribunal order the reduction of an assessment, the Comptroller shall reduce the assessment accordingly, without limit as to time.
(3) Where, for any year of income a person who has furnished a return of income for that year and has been assessed under sections 97 or 98 notifies the Comptroller in writing within 6 years after the end of that year of income that by reason of some error or mistake of fact in such return the assessment was excessive, the Comptroller after taking into account all relevant circumstances and subject to subsection (4), may reduce the assessment to provide such relief as appears to him or her to be fair and reasonable.
(4) No relief shall be given under subsection (3) if the assessment was properly made in accordance with the practice generally prevailing at the time the return of income was made.