3. Where two or more properties within the same curtilage, or contiguous to one another are in the same occupation and, though treated for any reason as two or more hereditaments for the purposes of valuation and rating, are used as parts of a single mine, quarry or factory, then, for the purposes of determining whether the several hereditaments are industrial hereditaments, they shall be treated as if they formed parts of a single hereditament comprising all those hereditaments.N.I.