That never happened.
http://urbanlegends.about.com/od/humor/a/bristol_zoo_car_park_attendant.htm
Analysis: If ever there was a story too good to be true, this is one. Not only has a crack team of journalists from the
Bristol Evening Post conducted a thorough investigation and found the tale of the phantom car park attendant to be "nothing more than an urban myth," they have also succeeded in pinning down its exact point of origin: the
Bristol Evening Post!
And I quote: "A version of the story did appear in the
Evening Post two years ago," explains an article in the June 13, 2009 edition of the paper, "in a feature on urban myths published to coincide with April Fools' Day."
I.e., it's a two-year-old April Fools' prank gone viral; nothing more to it than that. For the record, the article further states that Bristol Zoo actually has more than one car park — several, in fact, none of which are open to coaches (buses) — and a comparable number of duly-hired attendants on the job.