Found it!!! In the same field as Y Garreg Goch standing stone.
Follow directions for Y Garreg Goch. When you enter the field look to your left and you will see the top of a roof and chimney of a house over the field hedge. Scan across to the right, along the hedge row and you will see a single large tree. Just to the right of the tree, in the hedgerow (behind the barbed wire fence) completely covered in ivy is Pistyll Gwyn. It would have been impossible to have spotted this stone in the summer. I only just noticed it because its silhouette stood out through the hedge. Even then I had to peel some of the ivy off to confirm it was a standing stone underneath and not an old tree stump! Felt very pleased with myself!
The stone is about the same size as Y Garreg Goch.
When we visited we could not find this stone but did find its partner the nearby Garreg Goch
Coflein says this of Pystyll Gwyn;
An erect monolith, 1.9m high by 1.0m by 1.1m deep.
(source Os495card; SN41NE2)