May to July. All over central London. 260 hand painted elephants in aid of real ones. Where ever you go you only need to spot them (not that difficult - tourists and children are crawling all over them :D)

I found one by Hilton Tower Bridge,

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by the  City Hall

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Insert Insert in Hay's Galleria,
Insert by the Royal Exchange
Insert by the Bank of England
Insert one more in the City
Insert by St. Paul's
Insert by National Theatre
Insert on the South Bank
Insert Insert Insert at Leicester Square
Insert at Trafalgar Square

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and in many other places...