I know Letchworth is famed for it's black squirrel population, can't find many links about them, but the NHDC website had this to say:
"...these areas house Letchworth's famous black squirrels. First sighted in 1944, these truly black squirrels are found only within four miles o f the town centre. They have featured on national television on several occasions, and a local public house is named after them."
(EDIT - lots of information on other sightings here too)