“(1) If an officer of the Board or the Board discover, as regards any person (the taxpayer) and a year of assessment— (a) that any income which ought to have been assessed to income tax, or chargeable gains which ought to have been assessed to capital gains tax, have not been assessed, or (b) that an assessment to tax is or has become insufficient, or (c) that any relief which has been given is or has become excessive, the officer or, as the case may be, the Board may, subject to subsections (2) and (3) below, make an assessment in the amount, or the further amount, which ought in his or their opinion to be charged in order to make good to the Crown the loss of tax. (2) Where— (a) the taxpayer has made and delivered a return under section 8 or 8A of this Act in respect of the relevant year of assessment, and (b) the situation mentioned in subsection (1) above is attributable to an error or mistake in the return as to the basis on which his liability ought to have been computed, the taxpayer shall not be assessed under that subsection in respect of the year of assessment there mentioned if the return was in fact made on the basis or in accordance with the practice generally prevailing at the time when it was made. (3) Where the taxpayer has made and delivered a return under section 8 or 8A of this Act in respect of the relevant year of assessment, he shall not be assessed under subsection (1) above— (a) in respect of the year of assessment mentioned in that subsection; and 3 (b) in the same capacity as that in which he made and delivered the return, unless one of the two conditions mentioned below is fulfilled. (4) The first condition is that the situation mentioned in subsection (1) above is attributable to fraudulent or negligent conduct on the part of the taxpayer or a person acting on his behalf. (5) The second condition is that at the time when an officer of the Board— (a) ceased to be entitled to give notice of his intention to enquire into the taxpayer’s return under section 8 or 8A of this Act in respect of the relevant year of assessment; or (b) informed the taxpayer that he had completed his enquiries into that return, the officer could not have been reasonably expected, on the basis of the information made available to him before that time, to be aware of the situation mentioned in subsection (1) above. (6) For the purposes of subsection (5) above, information is made available to an officer of the Board if— (a) it is contained in the taxpayer’s return under section 8 or 8A of this Act in respect of the relevant year of assessment (the return), or in any accounts, statements or documents accompanying the return ….”
“The test to be applied, in my view, is to consider what a reasonable taxpayer, exercising reasonable diligence in the completion and submission of the return, would have done.”