Last week I collected a few samples from the gravelly pathside on the approach to the barrage in Cardiff Bay. It was raining so I just stuffed a few samples in bags at intervals along the path (an approach I often use in urban areas where there are too many people around to loiter on hands and kness with a hand lens).
Checking them under the microscope later, as well as an abundance of Bryum ruderale were two samples of what I think is Brachythecium albians - they have the classic upright stringy shoots of that species and look spot on microscopically. The odd thing is that both samples has sporophytes - note very immature sporophyte in centre of top photo, as well as the old sporophyte on the left in the lower photo.
So, this would appear to be quite an unusual observation...unless I've made a howler with the ID! The seta is smooth which would rule out most other Brachythecium.