"(4) Where the appeal is against a decision with respect to any of the matters mentioned in paragraphs (b) and (d) of section 59(1) above it shall not be entertained unless- (a) the amount which the Commissioners have determined to be payable as tax has been paid or deposited with them, or (b) on being satisfied that the appellant would otherwise suffer hardship the Commissioners agree or the tribunal decides that it should be entertained notwithstanding that that amount has not been paid or deposited. ... (6) Where on an appeal under this section it is found that the whole or part of any amount paid or deposited in pursuance of subsection (4) above is not due, so much of that amount as is found not to be due shall be repaid with interest at such rate as the tribunal may determine."