Where under this Act any difference (other than a difference which falls to be determined by the F1Upper Tribunal) is to be determined by arbitration, then, subject to any other provision of this Act, the difference shall be referred to, and settled by, a single arbitrator to be agreed between the parties or, in default of agreement, to be appointed on the application of either party, after notice in writing to the other, by the President of the Institution of Civil Engineers.