I dithered for a bit over this one, there was no easy access so I bit the bullet and asked for permission, glad I did so too.
I think the farm house was called Tir Bach aswell and the gates were tall and a bit hard to open, I walked amongst the curious chickens and knocked on the door, and again and again, hmm what to do ? In the near distance I could hear a tractor so I headed for that and bingo, I found the farmer and his wife digging a big hole in the corner of the field and there was the stone in the other corner. With them were two terrier type dogs which were now barking at me. The farmer saw me and came over so I asked if I could see their stone.
I think they were quite surprised that someone wanted to photograph their stone and they relayed this to me about it, Three boffins from CADW came over to inspect the stone after the farmer had told them of other boulders unearthed nearby, they said they were unrelated to the stone but then the boffins argued a bit about the stone and this was the gist, the menhir was in upside down but had been so for millenia on the western face of the stone was two small lumps of quartz with rings carved round them, and on the north face is an inordinate amount of recentish carved graffitti initials and dates mostly. One thing I noticed is the north face is really flat and precisly aligned east-west. It was about 6.5 ft tall and the western view is not visible due to high river bank