Confidence: I'm as sure as I can be.
Notes: The thalli (crusts) are covered by powdery, green soredia and the general effect is like a Lepraria, except that there are distinctly delimited colonies here. There are also sorediate Lecanora species that look like this. However, the giveaway is that the right hand photograph, at highest resolution, shows the presence of small, black, 'lecideine' apothecia. So Lecidella scabra, problem solved. Quite common on walls, more so in the south I think. Not on my own site yet as my photographs are uniformly rubbish.