“It is implicit in the powers given to the general or special commissioners to give a direction requiring the issue of a closure notice, and as part of that ‘reasonable balance’, that a closure notice can be required notwithstanding that the officer has not pursued to the end every line of enquiry or investigation - what is required is that he should have conducted his enquiry to a point where it is reasonable for him to make an informed judgment as to the matter in question, so that, exercising such judgment, he can state his conclusions and make any related amendments to the taxpayer’s return. The exercise of that judgment may require the officer to express his conclusions in broad terms…”