I wondered if that's what you were referring to. I don't think the problem here is that far up the stream, as the mains Earthing in the house has been in situ since I moved here, some 14 years ago and has been tested in anger IIRC at various times with short circuits and fuses blowing but I get that you're covering the bases and going to the worst case scenario, identifying the source.

As I've since posted... I've narrowed it down to certain wall sockets in the utility cupboard... I think