Confidence: It's likely to be this, but I can't be certain.
Notes: It can be difficult to tell this lichen from the free-living alga, Trentepohlia umbrina, without microscopic examination. I am fairly certain that T. umbrina is the algal partner in Belonia in any case. Looking at the photographs at highest magnification I think there is enough structure to suggest that this is indeed the lichen, but I would not record it without further checking. Both Belonia and T. umbrina are frequent on limestone gravestones.