We spotted apothecia on a Parmelia-type licen on acid gritstone, Peak District. I don’t think I’ve ever seen apothecia on Parmelia in the Peak District.
The reddish-green colour of the apothecia puzzled me as I know Parmelia usually have red-brown apothecia. However, it had been wet and I wonder if the algae were dominating the fungal colour.
Looking at Dobson’s (2005) key:
Thallus grey-green when dry. Upper surface K+ yellow (when absorbed onto tissue).
With isidia (yes).
White pseudocyphellae as white lines and dots. Coralloid isidia. No marginal cilia (yes).
This gives Parmelia saxatilis ( P. ernstiae excluded as it has a pruinose upper surface).
Simple or rarely squarrose rhizines (these look more furcate).
Apothecia lecanorine dark red-brown (these are reddish green) margins thin and frequently isidiate or crenulate (yes).
Spores simple 16-18 x 9-11 (these are a little smaller). (No reaction with KOH and then Lugol, but I only added Lugol once - not 3 times as in Dobson

Paraphyses look as in Dobson's diagram.
Medulla K+ red/orange (yes).
I presume this is fertile Parmelia saxatilis.
Steve