I collected a sample from a sunny concrete wall at Pencoed Rugby Club (of all places) today. It looks good for Didymodon rigidulus in all respects except that many of the leaf tips are missing (and they look like they've been shed rather than chewed off by slugs). Is this species known to lose its leaf tips? I can find no mention of it in the books I have.
I'm pretty sure it's not Didymodon sinuous as the margins are recurved (and entire) and the nerve is very strong.