3A person to whom a copy of a draft has been delivered under either of the two last preceding paragraphs may within six weeks after the delivery thereof serve notice on the Commission of his objection to the terms thereof on the ground that the proposed lease does not comply with the provisions of section thirteen of this Act, or that the proposed substituted security or other instrument does not comply with the provisions of the last preceding paragraph, as the case may be, and where notice is so served, the terms of the draft may be modified by agreement between the parties concerned, and in default of agreement the terms thereof shall be settled by an arbitrator selected by agreement between the parties or, in default of agreement, by the Lord Chancellor in the case of England or the Lord President of the Court of Session in the case of Scotland.