I dropped in to Craig y Cilau NNR recently and happened to walk across an area of the raised bog that I may not have visited before - in a few small runnels were some nice patches of Marchantia polymorpha ssp. montivagans. I have only seen it a couple of times - there is a small cluster of sites in the Crickhowell area. I had a quick look at the NBN Gateway to get an idea of how many records there are in south Wales - unfortunately there seem to be quite a few BIS data inputting errors muddling the true picture.
I looked hard for this subspecies when recording in Carms - I once managed to convince myself that some material I found near Usk Reservoir was montivagans, but on drying it developed a black median line and turned out to be just typical ssp polymorpha! When I saw the real thing for the first time it was quite striking - it is a much chunkier plant than polymorpha, with thalli almost twice as wide and rather thick and leathery. A couple of pics below - unfortunately the plants had been frosted the night before, so not looking their best.
On a nearby willow was a single tuft of Ulota calvescens, which I think is new for the reserve and probably the furthest south-east I have seen it in Brecks.