After perusing Magic and the Derbyshire SMR I think its an 18th/19th century spoil heap which is both a shame and a relief. Still very odd and worrying that someone is taking chunks out of mounds on the moors in the peaks.

On a related topic, turns out there should be 3 cup marked stones inc a cup and ring dotted about on that raised shelf. Something to look for when I'm back looking for the mysterious circular structure.