43.—Whenever the Board, by an alteration made under this Act of the area of supply of an authorised undertaker, extends such area (whether the Board does or does not at the same or any other time in other respects reduce such area) the following provisions shall have effect, that is to say, if the extent of the area so added to such area of supply exceeds one-tenth of the extent of such area of supply before it was so altered, such authorised undertaker may within one month after the date of the order of the Board making such alteration require the Board to acquire the undertaking of such authorised undertaker and thereupon the Board shall by order do whichever of the following things it thinks proper, that is to say, either acquire such undertaking or take control of such undertaking for a period of not less than two years.