This section applies where an enquiry into a return is in progress in relation to any matter as a result of notice of enquiry by an officer of the Board under section 9A(1) of this Act.
If the officer forms the opinion—
(a) that the amount stated in the self-assessment contained in the return as the amount of tax payable is insufficient, and
(b) that unless the assessment is immediately amended there is likely to be a loss of tax to the Crown,
he may by notice to the taxpayer amend the assessment to make good the deficiency so far as it relates to the matter.
In the case of an enquiry which under section 9A(5) of this Act is limited to matters arising from an amendment of the return, subsection (2) above only applies so far as the deficiency is attributable to the amendment.
For the purposes of this section the period during which an enquiry is in progress in relation to any matter is the whole of the period—
(a) beginning with the day on which notice of enquiry is given, and
(b) ending with the day on which a partial closure notice is issued in relation to the matter or, if no such notice is issued, a final closure notice is issued.