(1)An assessment to tax made by virtue of—
(a)section 36 of this Act, or
(b)so far as they relate to an assessment for a period ending not earlier than six years before the end of the normal year or normal accounting period, section 37, 38 or 39 of this Act, or
(c)section 40(2) of this Act,
may only be made with the leave of a General or Special Commissioner given on being satisfied by an inspector or other officer of the Board that there are reasonable grounds for believing that tax has or may have been lost to the Crown owing to the fraud or wilful default or neglect of any person.
(2)The General or Special Commissioner giving leave to make such an assessment shall take no part in the proceedings, and shall not be present, when any appeal against the assessment is heard or determined.