It's a long slog across the Breconshire moorland to get to Llech Sychryd on the Nant Hir (SN991073), but this appeared from the OS map to be a potentially interesting waterfall (with HEP potential so needing bryological exploration) so it needed to be checked. Was it worth it?...
No, not really. I noted 78 species on the east bank, but most were commonplace bryophytes of wooded valleys. Highlights were Jungermannia obovata on a seeping shale rockface at SN99190736 - initially misidentified as J. sphaerocarpa but upon checking found to have a perigynium - and Neckera crispa & Gymnostomum aeruginosum on a slightly base-enriched outcrop at SN99060728, but the area behind the dramatic waterfall was very base-poor. Supporting cast included Heterocladium heteropterum, Saccogyna viticulosa, Fissidens celticus, Lejeunea lamacerina, Gymnocolea inflata, Jungermannia pumila, Bartramia pomiformis, Ditrichum heteromallum and Sphagnum squarrosum. I mustn't grumble, but I'd hyped the place up in my mind and walked 3km to get there. The massive rock slab through which the waterfall cuts was really spectacular, but sadly it was in VC42 and I didn't have time to make a list.
The tetrad list was bumped up by a few ruderal species on the Nant Hir Reservoir dam, including Bryum pallescens, Pseudocrossidium hornschuchianum and extremely inland Syntrichia ruraliformis. There appears to be just one previous VC41 record from the tetrad - Barry's Rhytidiadelphus squarrosus from Nant-hir viaduct - so my 99 species are almost all new.