In this Part of this Act, " existing discharge " means a discharge of trade effluent from trade premises into the sewers or sewage treatment works of a local authority which was lawfully made within the period of two years ending on the date of the commencement of this section:

Provided that where before the said date the local authority and the person making the discharge have agreed that after that date the nature or composition of the discharge may be altered or the temperature, volume or rate of discharge may be increased, any discharge made in accordance with such agreement shall be treated for the purposes of this Part of this Act as an existing discharge.

Any dispute between the local authority and the person making the discharge as to whether the discharge is an existing discharge shall be determined by the sheriff, against whose determination an appeal shall lie to the Court of Session.