Hi I'm guessing, but maybe Dyer is thinking that a flash mob must materialise out of the crowd briefly and then, just as suddenly, disappear again. If it goes on too long, it is a performance, which is a different thing Come to think of it, the clue's in the title: flash mob Dave :-)
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red appleLiverpool Street ... liverpool streetHow is it that you are able to capitalize correctly when quoting from a text, but not when you write your own question about it?