Lichen (Xanthoria parietina)
Lichens may not look much, just some dried up crusty old vegetation, but they are actually fascinating. A lichen is actually two living organisms, an algae and a fungus, which live together for mutual benefit, symbiosis (I'm turning into Chris Packham!)
To survive they need a host which may be vegetable or mineral from which it can derive support, minerals and moisture. Lichens do no harm to their hosts, they are not parasitic.
Identifying lichens is a real headache. This one though is very easy as it is really the only yellow coloured one and it is extremely common, mainly because it seems to be resistant to air pollution.
You can find Xanthoria on trees, rocks and walls, especially on bird perching sites such as fence posts and milestones. Unfortunately they do not have English names.