* I patched the wall up and have marked the location of the grey cable
with an arrow scratched on the wall. Is there a standard symbol for
marking the location of electrical mains cables (E, perhaps?)

* Is it possible someone used wooden trunking for the electric cables
prior to plastering?

Bruce

I would just write with a felt tip pen in large letters 'CABLES HERE'.
Should be obvious then!!!!

Dave

--
For what we are about to balls up may common sense prevent us doing it
again
in the future!!