Where by reason of absence from the United Kingdom any party is prevented from treating, or cannot after diligent inquiry be found, the amount of such compensation shall be determined by valuation in manner following; that is to say, the said Secretary of State shall make application to two justices; and upon proof satisfactory to them that any such party is by reason of absence from the Kingdom prevented from treating, or cannot after diligent inquiry be found, such justices shall, by writing under their hands, nominate a competent surveyor for determining the amount of such compensation as aforesaid; and such surveyor shall determine the same accordingly, and shall annex to his valuation a declaration in writing subscribed by him of the correctness thereof.