Found on grassy hillside near Craig Cerrig Gleissiad.
No interactions present.
Also C. acaule is a plant of limestone soils. If I remember my geology correctly this observation is from the Old Red Sandstone.
I am not a geologist, but the area does indeed have a mix of Limestone and Old Red Sandstone. Not sure which dominates at this location.
Given feedback, am content to accept that "my specimen" is Spear Thistle rather than Dwarf.
The main body of the high ground of the Brecon Beacons is composed of Old Red Sandstone. I've looked at a map, and the Carboniferous Limestone outcrops as a narrow belt on the south side, starting just north of Merthyr - i.e. well south of your observation.
Many thanks; have also researched the substrate in the area and agree that it is the mixture of materials known as Old Red Sandstone. Looks like Dwarf Thistle is ruled out.
Lat/Lng: 51.8842, -3.5037
OS grid ref: SN965216