3. The amount of a motor vehicle allowance payable at the casual user’s rate shall, in any year, be an amount calculated, in relation to the mileage of authorised use in that year, at such rate as is specified by the Secretary of State by reference to that mileage and the cylinder capacity of the motor car in question, except that where the amount of the allowance would be less if it were calculated at the essential user’s rate, it shall be of an amount calculated in accordance with paragraph 2.