If a part only of any such mortgaged land be required for the purposes of the special Act, and if the part so required be of less value than the principal money, interest, and costs secured on such lands, and the mortgagee shall not consider the remaining part of such lands a sufficient security for the money charged thereon, or be not willing to release the part so required, then the value of such part, and also the compensation (if any) to be paid in respect of the severance thereof or otherwise, shall be settled by agreement between the mortgagee and the party entitled to the equity of redemption of such land on the one part, and the promoters of the undertaking on the other; and if the parties aforesaid fail to agree respecting the amount of such value or compensation the same shall be determined as in other cases of disputed compensation; and the amount of such value or compensation, being so agreed upon or determined, shall be paid by the promoters of the undertaking to such mortgagee in satisfaction of his mortgage debt, so far as the same will extend; and thereupon such mortgagee shall convey or release to them, or as they shall direct, all his interest in such mortgaged lands the value whereof shall have been so paid; and a memorandum of what shall have been so paid shall be endorsed on the deed creating such mortgage, and shall be signed by the mortgagee; and a copy of such memorandum shall at the same time (if required) be furnished by the promoters of the undertaking, at their expence, to the party entitled to the equity of redemption of the lands comprised in such mortgage deed.