“34(1) Subject to the following provisions of this Act, and to any other provisions of the Taxes Acts allowing a longer period in any particular class of case, an assessment to income tax or capital gains tax may be made at any time not more than 4 years after the end of the year of assessment to which it relates. 34(2) An objection to the making of any assessment on the ground that the time limit for making it has expired shall only be made on an appeal against the assessment. … 36(1) An assessment on a person in a case involving a loss of income tax or capital gains tax brought about carelessly by the person may be made at any time not more than 6 years after the end of the year of assessment to which it relates (subject to subsection (1A) and any other provision of the Taxes Acts allowing a longer period). 36(1A) An assessment on a person in a case involving a loss of income tax or capital gains tax– … (b) attributable to a failure by the person to comply with an obligation under section 7, … may be made at any time not more than 20 years after the end of the year of assessment to which it relates (subject to any provision of the Taxes Acts allowing a longer period).”
“118(2) For the purposes of this Act, … where a person had a reasonable excuse for not doing anything required to be done he shall be deemed not to have failed to do it unless the excuse ceased and, after the excuse ceased, he shall be deemed not to have failed to do it if he did it without unreasonable delay after the excuse had ceased.”
“118(5) For the purposes of this Act a loss of tax or a situation is brought about carelessly by a person if the person fails to take reasonable care to avoid bringing about that loss or situation.”
“Whether acts or omissions are careless involves a factual assessment having regard to all the relevant circumstances of the case. There are many decided cases as to what amounts to carelessness in relation to the completion of a self assessment tax return. The cases indicate that the conduct of the individual taxpayer is to be assessed by reference to a prudent and reasonable taxpayer in his position: see, for example, Atherton v HMRC[2019] STC 575 (Fancourt J and Judge Scott) at [37].”