“The rebus sic stantibus rule identifies for the purpose of valuation the hereditament, the physical changes which may be made to it, and the mode or category of occupation. The rule rests on the concept that what has to be determined in rating is the value to the occupier of his occupation of the hereditament, measured by the rent on an assumed yearly tenancy. In carrying out a valuation under the rating hypothesis the following assumptions are to be made about the hereditament: (a) That the hereditament was in the same physical state as on the material day. Alterations which the hypothetical tenant might make to the hereditament may be taken into account if, taken overall, they are minor. All other prospective alterations to the hereditament are to be ignored. (b) That the hereditament could only be occupied for a purpose within the same mode or category of purpose as that for which it was being occupied on the material day. Any prospective change of use outside that mode or category is to be ignored…..”