...at least I hope it's a Weissia and I'm not barking up completely the wrong tree!
I found this at the weekend on a soil bank near Caerphilly Mountain. Associates were Fissidens taxifolius and Barbula unguiculata.
Unfortunately nearly all capsules are immature, but there is one which has lost its lid - see 2nd and 3rd photos below. This looks to have a narrow mouth suggesting brachycarpa. The leaves seem to have a mixture of plane and inrolled margins. Setae are about 2.5mm long.
Any thoughts? Thanks!