I'm sure I'm not the first to suffer from this terrible affliction!
I was down at Porthkerry with the family yesterday and after paying homage to the Southbya colony which Gareth found last year (which was very easy to find, on the first bit of dripping cliff after heading west from the beach) I collected some Bryum samples from the limestone cobbles at the back of the beach (general habitat photo below - bryophytes were mostly out of frame to the left of the shot).
These look to be two different species, but I'm confused by both of them. The first (photos below) has long, straight leaves with a longly exurrent nerve (untoothed) and prominent (often pinkish), recurved leaf borders. The basal cells are unicolorous with the rest of the leaf; rhizoids are brown and coarsely papillose and I couldn't find any tubers. I ran it down to couplet 32 in Smith, but at this point the lack of capsules left me stuck.
The second species at least had capsules, but attempting to key it out using Smith led to various unlikely possibilities. This moss also has short shoots but with much smaller leaves (approx. dichotomum size) that are strikingly concave. Leaves are obscurely bordered and with margins recurved usually on one side only. The nerve is excurrent and slightly toothed and the leaf bases are reddish. Again, rhizoids are papillose (but more finely so) and I couldn't find any tubers. Photos below.
Associates included Barbula sardoa, Hypnum resupinatum (I think - unusually small) and Homalothecium lutescens. I hope the photos and text will be helpful - any suggestions appreciated, thanks.