The books make out that Philonotis arnellii is a featureless moss that is easily confused with the very much commoner P. fontana. That is true to an extent, but it's one of those species that is really quite distinctive when you see the real thing: when you think you've found it, you've got P. fontana and it's only real P. arnellii when you know you've got something different. Most colonies I have found are on earthy ledges alongside rivers, perhaps flooded once every few years, but I've also seen it in the uplands on ledges and occasionally on flushed lane banks (perhaps like Barry's recent one). I haven't found convincing material on forestry tracks, where juvenile P. fontana is frequent and confusing.
Look for relatively dense patches of consistently very small shoots with very narrow leaves, expect there to be abundant deciduous branchlets (there usually are) and look for fertile buds (with even longer, narrower leaves) with a lens. A microscope is needed to check the cell mamillae: look at mature leaves on the lower, brown sections of the stem. Here are four photos from a large colony near Brechfa in central Carms.