I grew up just down the hill from Milk Churn Joan (as we were taught it by local farmers, etc). The story I heard as a kid was that Joan who had two very sickly parents went out in a very bleak and fierce winter in search of milk. I forget the details but she meets the devil who (perhaps) offers an exchange of life for the life of her parents. She accpets and that is her to this day on the hill.
As for donating a penny on top, this is indeed true and me and a friend would regularly go up there to stand on each other's shoulders to claim the cash. A local farmer (no doubt spuriously) presented a very large old whiskey bottle full of pennies claiming they had all come from the top of Milk Churn Joan to inspire further looting.
The stone is claimed to spin round three times on New Year's Eve. It is said to be named after a milk-maid who died whilst carrying milk to the villages here. There is also a legend that a penny placed on the stone will bring good luck.