2mm to 3mm leaves growing on old tree stump in damp wood.
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What makes it Rhizomnium and not a Plagiomnium? (I thought that the nerves went right to the tip of the leaves, which is why I opted for Plagiomnium, but it's a long time since I really looked at mosses.)
I'm only going by they keys in the BBS field guide! The shape of the leaf and the untoothed margin makes it key out quite easily
I admit I didn't use the key, I went to Mnium first, from a gut-response from a moss course many years ago, and checked nearby pictures - not very scientific I admit! But looking at the pictures, Plagiomnium rostratum (which is also untoothed) looks a slightly better fit - what do you think? The veins go right to the end of the leaves.
Revisiting the key, I see Plagiomniums can key out with untoothed leaves, if the stems are creeping. That's not clear from your pictures, but I'm now thinking things aren't as straight forward as I first thought. I'm not familiar with P. rostratum, but I would say in my defence that when I have seen Rhizomnium punctatum it does very much look like what you've photographed (for what that's worth!)
Lat/Lng: 51.73108, -3.75394
OS grid ref: SN789050