“15. The jurisdiction of the Special Commissioners is not limited to situations where the taxpayer claims to have been overcharged by a valid assessment. The jurisdiction covers situations where the taxpayer contends that there is no charge on grounds that the document purporting to be the assessment is invalid or ineffective. The most usual case is where the assessment is challenged as being out of time. Another example is where the taxpayer contends that the assessment is on the wrong person (e.g. where the assessment is on him as an individual whereas he claims he should have been assessed as a trustee). A further example of a challenge to the validity of the assessment that falls within the Special Commissioners' jurisdiction is where the taxpayer contends that the assessing officer did not have had the Board's authority to make the assessment. The words of s 50(6) do not, expressly or by necessary implication, restrict the scope of the appeal commissioners and prevent them from examining the validity of the assessment on those grounds. Indeed s 29(8) expressly provides for an appeal on the grounds that neither of the conditions in subsections (4) and (5) are fulfilled. 16. So here we hold that a person who has been assessed by the Director in pursuance of s 317 may put forward as one of his grounds of appeal that the person making the assessment, the Director, had no authority to do so on the basis that the Director has not satisfied the qualifying condition in s 317(1)(a). It would then be for the Director to show that she had properly served a notice on the Revenue under s 317(2) and thereby obtained the right to exercise the general Revenue functions specified in the notice. The jurisdiction of the Special Commissioners exists to entertain the appeal on those grounds without reference to any Human Rights or natural justice issues and without considering the implications of any general prohibition against retrospective legislation.”