"I do not think that anybody pretends that those figures are anything other than estimates or guesses. They are the best that the Revenue can do on the materials in front of them and they may very well, for ought I know, be a very poor approximation to the truth indeed. But the situation here is that once leave has been given to make the additional assessments and the additional assessments have been made, the onus is on the taxpayer to show that they represent over-assessments"