I attended the Glamorgan Botany Group excursion to Gwaelod-y-garth on Sunday, but had to leave at lunchtime. I therefore missed the best finds of the day, both Persicaria minor and mitis found growing by a seasonal pool near the Taff in the afternoon.
Julian Woodman brought back a clod of earth from the same area as he had noticed a crystalwort growing in abundance. This proved to be Riccia sorocarpa, but the clod also contained plants of Trichodon cylindricus, Tortula truncata and a tiny, fruiting moss which I can't work out - see photo below (the Riccia gives an idea of how small it is). The leaves are strongly toothed and translucent. The round, brownish capsules are on very short setae.
I'll get some microscope photos if it's not possible to work out the species from the above. Thanks.