Provided always, that no consent of any occupier of lands in such common arable fields, or of a separate sheep walk therein, to any such first agreement for the ordering, fencing, cultivating and improving of such common fields, to be made by the authority of or in pursuance of this Act, shall be good and valid unless such occupier shall, at the time of entering into such agreement, produce a written authority for that purpose, under the hand of the owner or proprietor, guardian or trustee, or in case of such owner not being a minor, and being in parts beyond the seas, of the known agent of such owner.