Diminution in market value to be considered in fixing compensation.

(1) In determining the amount of compensation under the last preceding section, regard shall be had to the diminution in the market value, at the time of awarding compensation, of the property in respect of which compensation is claimed. (2) Where such market value is not ascertainable, the amount shall be reckoned at twelve times the amount of the diminution of the annual net profits of such property, caused by the exercise of the powers conferred by this Act.