12In this Schedule " at a material date" means at either of the following dates, that is to say—
(a)the appointed day ; and
(b)the date by reference to which this Schedule falls to be applied in the particular case in question :
Provided that sub-paragraph (b) of this paragraph shall not apply in relation to any buildings, works or use of land in respect of which, whether before or after the date mentioned in that sub-paragraph, an enforcement notice served before that date has become or becomes effective.