15. The length of the loading space of a non-articulated vehicle that is located to the rear of the rear axle centre of the vehicle shall not exceed—
(a) in the case of a vehicle having an unladen mass that exceeds half the maximum gross mass of the vehicle permitted by this Act—a length equal to the length of the loading space forward of the rear axle centre of the vehicle; or
(b) in any other case—a length equal to 90 per cent of the length of the loading space forward of the rear axle centre of the vehicle.