I think it is safe to record your lichen as Lecanora sambuci. Other British lichenologists including myself have recorded such material under the name L. sambuci, based on specimens judged to belong to the L. dispersa group and having more than eight spores per ascus.
I checked what Lucyna Sliwa wrote about this species in her ‘Revision of the Lecanora dispersa complex in North America’, Polish Botanical Journal 52: 1-70, 2007. Her discussion is as follows:
“Lecanora sambuci is differentiated by having small apothecia with a thin, regular thalline margin and a brown disc, and by growing predominantly on branches and twigs of deciduous trees, preferring Sambucus. The following anatomical characters distinguish the species: (i) asci more than 8-spored, (ii) an epithecium not at all granular, and (iii) the absence of lichen products. The two most similar species are L. hagenii and L. persimilis. However, the number of spores is diagnostic, since L. sambuci is the only species of the complex having multispored asci. Additionally, L. hagenii has an often pruinose apothecial disc, and L. persimilis usually has a darker apothecial margin (often biatorine), sometimes concolorous with the disc.”
Some doubt is cast on the taxonomic significance of the multispored asci in her ‘Remarks’:
The multispored species of Lecanora s.str. were studied in detail by Guderley and Lumbsch (1999). The authors concluded that the number of ascospores is of minor taxonomic significance in the genus sensu stricto. Assessment of the significance of the character within the L. dispersa complex requires study of more material from Europe.”
Despite the caveat about how significant multispored asci are in Lecanora, for recording purposes we will be consistent if we continue to record material such as yours as L. sambuci. I suspect you are right that it is very overlooked, having a similar ecology to L. hagenii and being very similar in appearance to that species. However, I think it is genuinely rare in Eastern England. There is something about its look that has led me to collect it when I have been in other parts of the country but I don’t seem to find it in my home region.
