In subsection (3) of section 4 of the [1968 c. 7.] London Cab Act 1968 (which prescribes a penalty of £20 for a first offence and £50 for a subsequent offence under provisions of that section prohibiting the display or issue of certain signs, notices and advertisements in connection with private hire-cars) for the words " liable on summary conviction" onwards there shall be substituted the words " liable on summary conviction to a fine not exceeding £200 ".