15.—(1) Subject to sub-paragraph (2) the purposes for which the holder of a trade licence who is a vehicle tester may use a vehicle on a public road by virtue of that licence are the purposes of testing it or any trailer drawn by it or any of the accessories or equipment on the vehicle or trailer in the course of the business of the holder of the trade licence as a vehicle tester.
(2) The purposes prescribed by sub-paragraph (1) do not include the conveyance of goods or any other burden of any description on the vehicle except—
(a)a load which is carried solely for the purpose of testing or demonstrating the vehicle or any of its accessories or equipment and is returned to the place of loading without having been removed from the vehicle except for that purpose or in the case of accident, or
(b)a load which is built in as part of the vehicle or permanently attached to it.