Payment of additional Tax:— Where toy motor vehicle in respect of which

the tax has been paid is altered or used in such a manner as to causc the vehicle to become a vehicle in respect of which a higher rate of taxis payable, the owner thereof shall be liable to pay an additional tax of a sum whichis equal to the difference between the tax already paid in respect of such vehicle and the tax. which is payable in respect of such vehicle after ‘its being so altered or used.