I'll bet there are a few hits at this site to see what's going on.
Nelson's Monument?
OK. Its current location is not its original one. It was taken in 1805 from a site 1.6 km north where it was one of a number of similar stones in the field giving it a very probable prehistoric provenance.
It is now inscribed with a dedication to Nelson from the workers at Lorn Furnace.
It is a majestic piece of granite set, in its present elevated position, in a wonderful location against a backdrop of the surrounding hills.
Grinsell ('folklore of prehistoric sites in Britain') does not explain the folklore that goes with this site very much - but it sounds rather like your witch/giantess/devil + broken apron strings story: he says the stone was being carried by a (presumably rather large) old woman, and that it fell to the ground when one of her withies broke (withies are ropes made of long whippy willow twigs). (story recorded 1927).