I had heard that people from Hartlepool were nicknamed "Monkey Hangers", and wondered why.

You couldn't make it up.
From the article: "Unfamiliar with what a Frenchman looked like they came to the conclusion that this monkey was a French spy and should be sentenced to death."

Wiki has some more on this too.
There's even a song you can listen to.

Hartlepool also elected a monkey as Mayor in 2002.