4Where any property is acquired in accordance with this Schedule there shall be paid by way of compensation to the owner of the property concerned a sum equal to the price which he might reasonably have been expected to have obtained upon a sale of the property effected by him immediately before the acquisition of the property by the Secretary of State; and any dispute as to the amount of such compensation shall be determined by arbitration; and the compensation shall accrue due at the time when the property vested in the Secretary of State.