21.—(1) Subject to paragraph (2) below, the prior approval of the Board shall be required–
(a)for the employment in the Judicial Committee of the Privy Council or the House of Lords of counsel other than Scottish counsel;
(b)for the employment in the Court of Session of senior counsel or of more than one junior counsel;
(c)for the employment of counsel in the sheriff court, the Scottish Land Court, the Lands Tribunal for Scotland or the Employment Appeal Tribunal or in proceedings before the Proscribed Organisations Appeal Commission, the Social Security Commissioners or the Child Support Commissioners;
(d)for the employment of any expert witness; and
(e)for work of an unusual nature or likely to involve unusually large expenditure.
(2) Paragraph (1) above shall not apply where the Board, on an application made to it for retrospective approval for the employment of counsel, of an expert witness, or, as the case may be, for work of an unusual nature, considers that that employment or work would have been approved by it and that there was special reason why prior approval was not applied for.