We had a family outing to Kew Gardens on 26th June. I managed to resist the temptation to take any samples from the luxuriant bryophytes growing in the glass houses, reasoning that I'd have little chance of identifying any of them. I did make a small concession to bryology by taking a sample of a small acrocarp from damp stonework in the gardens, scraping it off using my fingernail.
Under the microcope the sample proved to be a mix of two species: young plants of Tortula marginata and T. muralis. The difference between the short, greenish hairpoint of marginata and the long, colourless hairpoint of muralis was quite striking, but otherwise the two looked similar - apart from the elongate, incrassate marginal cells of marginata, visible under high power (photos below).
The Atlas map suggests marginata is a widespread species in south-eastern England, but the only South Wales records away from the borders seem to be two from southern Pembrokeshire and one from Kenfig Castle - two of these records being made by Sam. One for me to look out for in Cardiff I suppose...Llandaff Cathedral might be a good bet.