When I saw this I assumed it was gracillimum, but it seemed to lack thickened leaf margins and attenuated shoots and just looked a bit different, so I collected a bit to check. Using Paton it tenuously keyed out as hyalinum - I'm not sure how convincing the bracts on the perianth are as a feature as gracillimum can show this to a degree it seems, but the presence of purple rhizoids is said to rule it out. I'm not terribly convinced this is hyalinum but as it would be new for the county I'd be grateful for any comments.
The habitat was the north-facing retaining wall (slag & sandstone) along the railway in the north of Swansea at Llwynbrwydrau (SS70039752). Most patches were growing on their own, but occasional associates included Pohlia nutans, Ceratodon purpureus, Cephaloziella stellulifera (photo below), Brachytheciastrum velutinum & Bryum sp. There must be a substantial population as I suspect this wall extends a long way east and west. This was a sneaky quick peak at a 5m section by my crossing point and it begs the question could the railway sections using slag be a potential habitat for Scopelophila?