I made the (near) big mistake of visiting tis site in the summer which made it even more difficult to find. An O/S map is a must although it is easy to park once you get to the edge of the forest. This has to be one of the most remote sites I have been to. The best tip I can give is that the burial chamber IS just about visible on your left. It is VERY low to the ground so you need to concentrate. It's only a short walk off the track but it took me a long time to find. Not much to see really but I guess it is satisfying when you do eventually find it!
I saw this on a map and told myself one day i'll come back and take a closer look, today was that day. Hopefully you'll have an os map to navigate the maze of small lanes, just follow the road untill it enters the forest, park here there's plenty of room. Take the path north for ten minutes then look out for big stone off to the left under the trees. The stone or 'Carreg' is a really good sized stone covered almost totally with mosses, resting on a slight bed of cairn , you can just see under the stone at a few points. There isn't much cairn material but the size of the stone makes up for it . Even the forest is light and breazy but obscures the view which would have been considerable.