91.Subsection (4) sets out the period allowed for beginning an enquiry. The period begins with the date on which a decision under section 18 is made. Where an estimate of income is given, this means the decision taken under section 18(6); otherwise, it means the decision under section 18(1) (subsection (5)). In the case of an enquiry relating to a person or persons, either of whom is required to make a self assessment tax return under section 8 of the Taxes Management Act 1970, the period ends on the day that tax return becomes final (or, if both persons make self assessment returns, the later of those two days) (subsection (4)(a)). A return becomes final when an enquiry into that return is completed or, otherwise, on the last day of the period during which an enquiry may be made into that return, as set out in section 9A(2) of the Taxes Management Act 1970 (subsection (7)). In any other cases, the period ends one year after the date by which a response to a notice issued by the Board must be made or, where an estimate of income is relied on initially, one year after the date the estimate must be corrected if it is incorrect (subsections (4)(b) and (6)).