10(1)If at a time when an enquiry is in progress into a return an officer of Revenue and Customs forms the opinion—U.K.
(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,
the officer may by notice in writing to the relevant person amend the assessment to make good the deficiency.
(2)If the enquiry is one that is limited by paragraph 9(2) and (3) to matters arising from an amendment of the return, sub-paragraph (1) above applies only so far as the deficiency is attributable to the amendment.
(3)For the purposes of this paragraph the period during which an enquiry is in progress is the whole of the period—
(a)beginning with the day on which the notice of enquiry is given, and
(b)ending with the day on which the enquiry is completed.