Botanical Notes, 1949-50
"Southbya nigrella (De Not.) Spruce. On calcareous tufa, in sheltered crannies of the cliffs between Aberthaw and Fontigary, 1949, and Porthkerry, 1950, associated with the mosses Weisia verticillata and Barbula tophacea and with the Maidenhair fern (Adiantum capillus-veneris), A.E.W."
Botanical Notes, 1960
"Southbya tophacea Spruce. Wet cliffs, Font-y-gary, 1949 and Porthkerry, 1950. Previously recorded as Southbya nigrella, but recently re-identified as S. tophacea and constituting the first record for that species in the British Isles."
Anyway, I couldn't resist a quick scoot along the bottom of Bull Cliff (ST0966). In the event the cliffs were soaking wet and there had been a lot of recent slumping, and I couldn't safely ascend off the beach. The slumped material contained only vascular plants. However, looking up, there did seem to be some tufa mounds high on the cliff (in the rubbish phone photo below the tufa was just below the horizontal band of scrub towards the left of the pic). But completely inaccessible.