If in the case of a licensed parking place which was ready for operation at the date of the grant of the licence the local authority are satisfied at any time that for a period of not less than two years beginning with that date the person, or each of the persons, who was for the time being during that period the holder of the licence has not made any significant number of parking spaces available to the public at the licensed parking place, then, subject to paragraph 13 below, they may by notice in writing to the holder of the licence revoke it.