On concrete fence
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Hi Jenny, have you looked at the colours of the apothecia in section?
With the well developed, areolate thallus and the somewhat convex apothecia with inconspicuous 'proper' margin, this looks rather like Clauzadea monticola to me.
Unfortunately I managed to lose my knife early that day so I couldn't check specimens of any of the saxicolous species - and they're the ones I need to learn ........
I wondered Clauzadea but the chemistry was wrong (specimen above tested K+ pale yellow) and though it had been raining there was no hint of reddish-brown to any of the discs.
Dobson - Lecidella stigmatea:
"Discs plane, occasionally becoming convex, very dark brown or black with glossy margins that become excluded."
Dobson - Clauzadea monticola:
"Apothecia black (dark red-brown when wet)"
But I've not (knowingly) come across Clauzadea monticola yet so I'm very open to correction!
"though it had been raining there was no hint of reddish-brown to any of the discs."
I think that indeed pushes the balance of probability strongly towards the Lecidella.
Lat/Lng: 51.6, -9.1
OS grid ref: SK1021