(1)No alteration shall be made in any part of the railway valuation roll until six months have elapsed from the completion of that part, or so long as there is pending any appeal the determination of which may necessitate the alteration of the amount stated in that part as the net annual value of the undertaking of the railway company as a whole, or, as the case may be, of the undertaking as a whole in so far as it is carried on in England; but on the expiration of" the said period of six months, or, as the case may be, so soon thereafter as every such appeal as aforesaid has been finally determined, the Railway Assessment Authority shall revise that part of the roll, by making such alterations therein as are necessary to give effect to, or are consequential on, any decision on appeal (whether or not such an appeal as is mentioned in this subsection) given up to that time and any decision of the Authority given up to that time upon a representation made to them under the next succeeding section :
Provided that, if no such alteration is necessary, the Authority shall confirm that part of the roll, and for the purposes of this Act such a confirmation shall be deemed to be a revision.
For the purposes of this subsection, an appeal shall be deemed to be still pending, and not to have been finally determined, until it is withdrawn, or until the Commissioners have given their decision thereon and either the time has elapsed for appealing against that decision, or any appeal against that decision has been withdrawn or decided.
(2)Except in accordance with the provisions of the next succeeding section, no alteration shall be made in any part of the railway valuation roll after it has been revised by the Railway Assessment Authority under the last preceding subsection other than such alterations with respect to any particular hereditament as may be necessitated by the decision of any appeal with respect to that hereditament which was outstanding at the date of the revision.