“[18] This should not be taken as an encouragement to officers of HMRC to draft every closure notice that they issue in wide and uninformative terms. In issuing a closure notice an officer is performing an important public function in which fairness to the taxpayer must be matched by a proper regard for the public interest in the recovery of the full amount of tax payable. In a case in which it is clear that only a single, specific point is in issue, that point should be identified in the closure notice. But if, as in the present case, the facts are complicated and have not been fully investigated, and if their analysis is controversial, the public interest may require the notice to be expressed in more general terms.”