If the amount in a sinking fund, together with the sums which will be payable thereto in accordance with the provisions of this Schedule, and also, in the case of an accumulating sinking fund, together with the accumulations thereon, will in the opinion of the Ministry be more than sufficient to repay within the fixed period the money for the repayment of which the sinking fund is formed, the council may reduce the payments to the sinking fund either temporarily or permanently to such amounts as will in the opinion of the Ministry be sufficient to repay within the fixed period the money for the repayment of which the sinking fund is formed.