Grimmia query

Here are a few photos of the Grimma referred to in the previous blog (see here). The sample is tiny but hopefully sufficient for a positive ID. It was growing on ORS either in or next to the rocky stream at SN971212. I've tried keying it out using Smith; the basal marginal cells are thin walled which limits the options to a few species. The long hair points (flattened at base, rounded apically) and upright capsule suggest G. donniana, but would this acidophile grow on ORS? The associates were mostly species suggestive of plenty of base enrichment (including Ctenidium molluscum as well as those mentioned in the previous post). The other confusing thing is I'm almost certain that when I collected it the seta was curved, but by the time I examined it the specimen had dried out a bit and the seta was straight (and no amount of rewetting will cause it to curve again). This might suggest G. arenaria, but as there are no South Wales records of that species it seems unlikely.

Hopefully someone more experienced with Grimmia can help; I can pass on the sample if necessary. Otherwise I'll have to call at the site next time I'm driving up the A470.