If at any time it appears to the council that the amount in a sinking fund, together with the sums which will be payable thereto in accordance with the provisions of this Schedule, and, in the case of an accumulating sinking fund, with the accumulations thereon, will not be sufficient to repay within the fixed period the money for the repayment of which the sinking fund is formed, the council shall, either temporarily or permanently, make such increased payments to the sinking fund as will cause the sinking fund to be sufficient for that purpose, and if it appears to the Ministry that any such increase is necessary, the council shall increase the payments to such extent as the Ministry may direct.