Having spent a fruitless hour looking in the area 200 yards to the east of the sheep pen as described by Bezant Lowe, I gave up and walked away disconsolately. Then lo and behold about 150 yards away I suddenly walked over exactly what I had been looking for all that time. I’m sure I only noticed it because I was grumpily walking along staring at my feet!
It turns out to be about 150 yards SE of the sheep pen right at the NE side of the Afon Ddu. It is 3 yards to the south of a granite pillar with the number 5 inscribed on it, which I have since discovered is a waymarker for the Llanfairfechan Upland Walk marked out by Conwy council.
The markings on the stone are beautifully clear.