Llyn Brianne: Stop-Off Point and Private Beach
As you drive around the reservoir, you will at some point pass through a wooded area. Look out for side turnings off to the reservoir's water edge. I chanced across this private bay and beach area, shown in this sequence of pictures, with excellent views down the reservoir. Much fun was had throwing sticks for the dogs, though on one occasion we had to watch out for a rather aggressive swan.
The area shown above comes complete with a sandy beach, and is clearly popular for the occasional picnic/ barbecue, as I found piles of stones in circles around several areas of burnt grass.
I imagine watching the sun set over this scenery would be very dramatic, but you are so far from anywhere, you would not want to miss the views on the rest of the drive around the reservoir by staying so late here.