I have found a much less soggy and strenuous walk and still with the outstanding views Postman speaks of.
If you are driving along the main A5 from Bethesda you will see a farm campsite on the opposite side of the road about 5 1/2 miles from Bethesda,
Just before you come to the Public FP marked on both sides of the road there is track with a gate and stile on the left hand side of the road. at approx SH 68806030.
This belongs to Welsh Water. If you park carefully close to the wall you will not be blocking the entrance.
Climb over the Stile and follow the tarmaced track up to the leat. At the leat there is a farm gate. Don't go through this but instead climb the stile on the right and follow the track around the edge of the leat.
You will have to cross over several very small stiles but the leat will lead you right round to the valley in which the stone sits.
Once you go around an obvious bend follow the leat to the weir/dam with the little hut beside it.
There is an obvious track leading down from here to the bottom of the dry stone wall. Here you'll find the stone and Cairn.
In Postmans 3rd picture he points out the dry stone wall but you can also see the line of the leat leading round the side of the hill.
A very long wet walk from the road with great potential for not finding owt. Fortunately I had plenty of time to wander about a bit. The footpath goes steadily uphill past a house and behind one till it gets to an attractive megalithic bridge. You can see where the standing stone is from here, look up to the mountain and come back down to a large brown bit and its there. Not much help, maybe, just follow the river with it on your left about thirty metres away or so and it'll take you straight there.
The stone is waist high with a rock pool on one side, the cairn is very close to it, and with some furtling about two or three fallen stones can be picked out. I'm inclined to beleive it was a circle, with awsome Tryfan as it's focus, perhaps when complete looking very similar to Hafodty stone circle some miles to the north east. A great place to enjoy Snowdonia, though maybe in a drought next time 'cause Welsh mountains are 50% water.