'What is the unit which the local authority are entitled to look at and deal with in an enforcement notice for the purpose of determining whether or not there has been a “material change in the use of any buildings or other land”? As I suggested in the course of the argument, I think that for that purpose what the local authority are entitled to look at is the whole of the area which was used for a particular purpose including any part of that area whose use was incidental to or ancillary to the achievement of that purpose.'