From the car park for Cwm George opposite Millbrook Road, walk into the woods. About halfway along the straight path (1 minute walk), turn left through a gap in the fence over a dry (hopefully!) river bed. [We now know the river will rise around October so you may have to access the crag from the back, see below for advice]. From the river bed, head up the track towards the castle and turn right after a minute or two over a large fallen tree. If you get to the castle walls you've gone too far, go back down and try turning right a bit sooner. Walk along the flat path through the woods for another minute and you will reach the rock on your left.
Access if the river is flooded: From Lettons Way, walk up the right hand side of a small concrete structure on the right. Follow the path through the woods until you reach the corner of the castle. Walk to the right and follow the perimiter of the castle around the next corner, you will pass an archway on the left. From here walk on the level roughly parallel with the castle walls, over the fallen tree mentioned in the above description and you will come across the rocks on your left. Keep a low profile if going this way especially if lugging boulder mats. We don't want to disturb neighbourhood watch.