2. Where any land is used temporarily or permanently for the exhibition of advertisements or for the erection of any structure used for the exhibition of advertisements and the land—N.I.
(a)is not so used under a right which is a separate hereditament from the land; and
(b)is not occupied for any other purpose;
the net annual value of the land shall be estimated by reference to the value of that use of the land[F1 unless but for that use it would be rateable under Article 25A].