"How about the song about the Monkey washed up on the beach and taken for a Frenchman?"
But that was true, wasn't it?? (joke)
It was usually used to describe the 'unworldliness' of people from Hartlepool. It dates back at least as far as the Napoleonic Wars and was still being told as fact during WW2.
My father was a building worker and he said that fights were often caused among the men (deliberately) by asking somebody from Harltepool, "Are you the man who hung the man who hung the monkey?"
Jim Carroll