“… must the demand be for x% of £y or can a simple demand for £z suffice? It can certainly be argued that the demand must specify the amount of the proportion to enable the lessee to understand the proportion which is claimed and to be in a position to dispute it so that it can be referred to the lessor's surveyor for decision. On balance, I prefer the construction which requires the lessor to state the sum which he demands (as a due proportion of its expenses) without separately stating the amount of the proportion and the amount of the expenses.”