I visited this site winter (it started to snow!) and couldn't find it. However, as I was about to give up, a couple were walking their dog and by luck they knew where it was. I can't really give directions other than to say it is in the middle(ish) of the common and is quite small - about 2ft x 10 inches. Only the top couple of inches are sticking up above the grass. Good luck!
Not just a cist, but a cist in the centre of a cairn circle (or very denuded kerbed cairn). And exciting finds were found there.
Eight stones, up to 0.5m high, define a kerb circle, within which is a cist, 1.6m by 0.6m.
Excavation, in 1830, which recovered burnt bone and 'bronze buttons' is thought to have removed the body of the cairn.