... well actually, 400m downstream from it, where I managed to squeeze in a quick smash and grab session in failing light (15:45-16:15). Although the water course and its banks seemed to support a rather limited bryo-flora, there were a couple of species of interest: Warnstorfia fluitans (locally frequent on the grassy flood zone alongside the stream) and Atrichum crispum, the latter conveniently growing adjacent to A. undulatum. Like the Warnstorfia it was mostly under water following the recent rains, hence no field shots. The species has a very different look to undulatum, with the shoots being more lax and a much brighter green. The photo below shows the narrowed leaf base and broad leaf shape. The ridges on the costa are smaller than those of undulatum, which you can get an impression of through a lens (just visible in photo above - you may need to zoom in a bit), which rules out any confusion with Mnium. This was only the third Glamorgan tetrad record, but given the rather mundane habitat, it's one we might expect to see more of in upland streams.
Warnstorfia fluitans with what looks like rather large alar cells, but the habitat, habit (dense sprawling mat) and presence of rhizoids on the leaf tips (with algae entangled) all make a safe id. Note the denticulate margins rule out Drepanocladus.