This site looks very similar to Black Coppice Chambered Cairn but still buried with a similar sized cap stone fully visible but only a small gap visible of the possible chamber.
Unfortunately the photograph I took came out corrupted. This may have been the cold getting to it internals or something else !!
I suggest this site is seen at its best during the winter months as summer vegetation will obscure it.
This stone and others like it on the plateau are considered to be millstone roughs propped up with little stones. (Anglezarke and Rivington Moors - Archaeological Survey 1986)