8(1)In subsection (1) of section 33 of the Management Act (error or mistake)—
(a)after the words “an assessment” there shall be inserted the words “(whether under section 9 or 11AA of this Act or otherwise)”; and
(b)for the words from “six years” to “made” there shall be substituted the words—
“(a)in the case of an assessment to income tax or capital gains tax, five years after the 31st January next following the year of assessment to which the return relates; and
(b)in the case of an assessment to corporation tax, six years after the end of the accounting period to which the return relates,”.
(2)The proviso to subsection (2) of that section shall cease to have effect and after that subsection there shall be inserted the following subsection—
“(2A)No relief shall be given under this section in respect of—
(a)an error or mistake as to the basis on which the liability of the claimant ought to have been computed where the return was in fact made on the basis or in accordance with the practice generally prevailing at the time when it was made; or
(b)an error or mistake in a claim which is included in the return.”