Notwithstanding anything in the Electricity (Supply) Acts, 1882 to 1919, or in this Act, or in any other Act of Parliament, or any Provisional Order or Special Order made under any such Act, a person shall not he entitled to demand or continue to receive for the purposes of a stand-by supply only from any authorised undertakers a supply of electricity for any premises having a separate supply of electricity or a supply (in use or ready for use for tie purposes for which the stand-by supply of electricity is required) of gas, steam, or other form of energy unless he has agreed with the undertakers to pay to them such minimum annual sum as will give them a reasonable return on the capital expenditure incurred them in providing such stand-by supply and will cover other standing charges incurred by them in order to meet the possible maximum demand for those premises. The sum to be so paid shall be determined in default of agreement by arbitration.