Going through some samples collected from a visit to Creigiau at the weekend, I wondered to myself 'why did I bring that Hypnum home?'. Then I looked at the capsules and realised it was Pylaisia polyantha. In the field I hadn't bothered to look properly at the capsules (and hadn't noticed there were several generations of capsules present), but I guess something about it looked different from typical Hypnum.
The habitat was a willow trunk in wet woodland at ST076812 - only about 3km north-west of where Sam found it recently in similar habitat (at Ty-du SSSI), but in a different 10km square. It is likely that I've overlooked other populations of this moss in south-east Glamorgan.