Caloplaca ruderum (images of British lichens)

Thallus thick, rather pale ochre-yellow, irregular and lumpy, with convex, pruinose areoles and lacking well-defined marginal lobules; apothecia urn-shaped when young, becoming flat or only slightly convex, discs brownish orange, thalline margins thick, pruinose. Generally on limestones and mortar, frequent only in south-east England and East Anglia, very local on base-rich and generally coastal rocks elsewhere.
Refs: Smith et al. (2009), 270; Purvis et al. (1992), 156; Dobson (2005), 106 (photo); van Herk & Aptroot (2004), 104-105 (photo); Lichen Atlas of the British Isles 6: 275 (2001).

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