JonasRavn
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For what it’s worth, I have collected several specimens similar to yours from exposed faces of roadside trees and concrete in Denmark, in the hope that they might be P. sciastra and so far I have concluded that they are all forms of P. orbicularis. In more well developed specimens, it is usually possible to see orbicularis-like circular soralia with quite granular soredia, that eventually develop cortex and appear isidia-like. It is my understanding that the isidia in P. sciastra are mainly marginal, so the presence of the “isidia” in circular laminal/submarginal patches might be a way to rule out P. sciastra. I am not an expert in the genus, but I have looked at hundreds of thalli in the past year, trying to come to terms with the bewildering variability of P. orbicularis. I was lucky to have a few of my collections sequenced, one small-lobed morph with terminal capitate soralia and one with long marginal soralia and marginal rhizines, but so far everything has been orbicularis! 
I might be wrong – determining wether something is a lichenicolous fungus or the pycnidia of the “host” is a constant struggle for me. I have spent more time than I care to admit on lichenicolous fungi that turned out to be nothing more than host pycnidia Hi Mark and Alan, First of all, thank you for your replies.
C. contorta ssp. hoffmanniana was actually one of the two IDs I considered (the other being C. caesiocinerea), but the habit was very different from the other examples of this taxon I have seen, so I rejected it at first, but perhaps I should reconsider.
There is no reaction with K – neither in section or from spot tests directly on the thallus.
I have tested the rock with different acids with no visible reaction. There are a few calcareous rocks in the stone wall, so perhaps runoff from these is what is causing the “shift” in substrate.
I don’t remember offhand if any of the calcareous rocks were directly above the sandstone, but I think that it was one of the rocks on top of the wall. The spore count and shape fits with C. contorta spp. hoffmanniana, so if nothing else comes of this discussion, I will tentatively name it as such
All the best,
Jonas
If it helps, I can add that the species was quite abundant on the east-facing surfaces of old Pinus sylvestrisstanding at the edge of a forest and thus exposed – about a kilometer from the nearest coast. Good luck on the search! Thank you Neil for the reply! I am convinced that you are right – the fit is very good. I am glad to put this troublesome specimen to rest!
All the best,
Jonas
I made an k/ink/vinegar-stain and found that while some paraphyses were simple, branching and anastomosis was common: 
[attachment=2]Billede3080.jpg[/attachment] And much to my surprise, some spores appeared one-septate (I found three out of ~50 spores like this):
[attachment=1]Billede3081.jpg[/attachment] Lastly a picture of the excipular hyphae:
[attachment=0]Billede3083.jpg[/attachment] The thallus might have an extremely faint UV+ orange reaction (only barely visible with powerful torch in pitch black room, xanthone?), but it is too weak to capture on a photograph.
I have tried keying it as a Lecidea without much luck – it shares some characteristica with L. hypopta, but it is not a perfect fit. I cannot rule out L. exigua, but the paraphyses do not appear capitate above which causes some hesitation.
The Revision of British and Irish Lichens vol .11 (Ramalinaceae) points me in the direction of Biatora, but it does not key out to anything meaningful.
I hope these new pictures might shed some light on the true identity of my specimen.
Thank you for your replies, Alan and Mark. I have been/still am busy with field work, but tonight I had some time to follow up on your input.
I am not much closer to an ID, but your suggestions prompted me to do some more testing/staining that might help.
First of all, some apothecia have a bluish pigment in the excipulum (consistent with the Lecidea-hypothesis?):
[attachment=2]Billede3070.jpg[/attachment] The brown epithecial pigment reacts N+ red and the crystals are not soluble in N:
[attachment=1]Billede3072.jpg[/attachment] 
[attachment=0]Billede3074.jpg[/attachment] Thank you for your great effort in confirming this ID! I have rechecked my material and made some follow-up photos with scale-bars attached.
From what I can see, the algae fit fine: they are rather small and generally look as in your attached photo. Furthermore, it looks like they form in vertical “rays” in thin sections.
A few photos of the algae:
[attachment=4]Billede2990.jpg[/attachment] 
[attachment=2]Billede3000.jpg[/attachment] 
[attachment=1]Billede3001.jpg[/attachment] The algal units seem to fit in size – although a few exceed the range (~350 µm). These are very oblong units so perhaps it is ok?
The size of the units may be judged from this photo. The perithecium in the middle is the one from which the sections in the following have been made.
Note: there’s a small black pycnidium or perithecium (no contents found so far), which I am certain must belong to a lichenicolous fungus.
[attachment=0]Verrucaria cf. ochrostoma 1-5.jpg[/attachment] The hamathecium is without interascal filaments (although with “characteristic” Verrucaria-inclusions), but with periphyses and/or periphysoids (not entirely sure how to tell the two apart if I am being honest)
[attachment=7]Billede2987.jpg[/attachment] 
[attachment=5]Billede2989.jpg[/attachment] 
[attachment=3]Billede2992.jpg[/attachment] For whatever reason, I still struggle finding mature and well developed spores. Most asci just contain shriveled spores or simply an amorphous substance. I did find two healthy looking spores in the last section, but they are clearly underdeveloped and rather egg-shaped rather than narrowly ellipsoid. The lack of good spores is my only cause for concern regarding the correct ID, but so much of the other characters fit very well, so I am inclined to overlook this.
[attachment=6]Billede2988.jpg[/attachment]
