Tried to find this site when visiting the nearby standing stone. As with Kammer I couldn't see it either. There are loads of large boulders about so perhaps it has been destroyed or burried beneath field clearance stones?
Visited 18th April 2003: I looked for this site with just a Landranger map, and couldn't find it. From the road I saw lots of stones that looked like they may have once been part of something, but it was all very inconclusive. More photogenic than these was a big pile of old cast iron baths.
Subsequently I've seen an old black and white photo of the site at the NMRW (sadly undated) showing the site to be a classic dolmen. What I can't be sure of is whether the site still exists, but from the write up I suspect it does, and I was just looking in the wrong place.
In Archaeologia Cambrensis 1872 (p139) they describe it as entirely demolished with only a few stones on the site. GE Daniel (1950) said only a few stones existed at that time, Barber & Williams (1989) report seeing the remains of a capstone and supporters in an overgrown hedge in 1985, and Children & Nash (1997) report the same, adding 'within a few metres of the tomb is evidence of extensive ground disturbance and a collection of rusting farm machinery'.
It's apparently also described in Royal Commission on Ancient Monuments No.1063 of Pembroke with a photo and description.
Children & Nash say that in 1830 the place was being levelled by labourers who discovered a neolithic polished stone handaxe made from gabbro, rock that had come from northern Ireland. Known as Tenby A8, it's now in Tenby museum.
Here's the account of the site that accompanied the photo at the NMRW. I think the record belongs to the Royal Commission:
In a field on Ffynon Druidion Farm are the remains of a small cromlech. The capstone, 5 and a half feet in length, 3 and a half feet in breadth, and 1 foot thick, lies north-west to south-west; so far as an observation was possible, it appears to be supported by two stout pillars. The immediate surface for some yards around is strewn with rough stone. About 600 yards due south, and within site of the cromlech, is an erect stone [the Ffynnon Druidion standing stone]...
It's not clear to me when the above was written. There were two dates on the paperwork, one relating to a visit to the site in 1920 and the other relating to a record made in 1872. The photo is most likely to related to the 1920 visit, so it seems likely that the site was still there in the early 20th Century. Hopefully it still is there.