“Not all uses of land or buildings fit within the use classes order. … Where land or buildings are beingused for different uses which fall into more than one class, then the overall use of the land or buildings is regarded as a mixed use, which will normally be sui generis. The exception to this is where there is a primary overall use of the site, to which the other uses are ancillary. For example, in a factory with an office and a staff canteen, the office and staff canteen would normally be regarded as ancillary to the factory.”