Growing on the thick bark of Quercus suber. According to the webside Lichens of Wales for Lecanographa amylacea (see the link above): "This is not one of the easiest lichens to identify with total certainty. Unless it is fertile - and I am not sure it has ever been found fertile in Wales -i t may be necessary to subject a small sample to thin layer chromatography to provide certain identification". I'm not able to make the TLC, but the photo at the lichen at the above cited website, the habitat (old trees in a fenced space), and the presence of species considered as similar, like Arthonia pruinata make me beleive that it can't be otherwise.