Thinking a little further through this I think I might be agreeing
with Andy Wade as cross connects of CPC's in a domestic situation are
fairly common as he describes.
What I was intending to do was use a steel adaptable box for the task
in my original post. Obviously this would need earthing, and if I just
used one of the CPC's to earth the box, then at some point in the
future someone could disconnect the lighting circuit, (for example),
at the CU and hence leave the steel adaptable box unearthed. - Any
more thoughts on THIS situation anyone ?