I have thought about the problem, and how I would with my tools locate what is causing it. It is not easy as you have no idea if lintel is neutral potential, earth potential or line potential all you know is it's not the same as your potential.
I would start with the survey already done. This should show earth type TN or TT which is a good start. I am assuming RCD protection all circuits which would limit line - earth current to 30 mA however with TT system unless double pole RCBO's are used one could still get a earth - neutral shock with RCBO protection.
We look at "wall or partition the internal construction of which includes metallic part, other than metallic fixings such as nails, screws" and forget the metal used by plasterers until we see cases like
Emma Shaw when I read the report I do wonder if I was the foreman would I have also taken the readings given to me as correct? But it does show how a fault can go undetected had there not been a water leak it could still be there today.
So using a volt meter simple enough to measure against the earth pin of a handy socket whoever showing voltage i.e. a fault is great, but if it shows no voltage then what? Point is proving something is dangerous is easy, proving it is safe is hard.
We should test the earth on every socket and every light switch and every ceiling rose. When I did my C&G2391 inspection and testing we were given a board to test about 5 foot square which took around an hour. If I was to test a house to the same degree it would take at least a day yet when my son was doing it for a living he was lucky to have 2 hours. And an hour of that was taken up writing down sizes of cables and cable types which in the main was a guess anyway. We talk about drive past inspection and tests and maybe not quite that bad, but until amendment 3 which I admit I have not read only been to talks, any where hard to access was missed.
I am told with amendment 3 even if it means removing tiles loft spaces should be inspected. Yes and that is really going to happen, I think not. Going silly means we have not a clue really what an inspection should include.
The fault may not even be in the house. Road works have been know to damage cables and cause voltage on the earth. Sorry not much help, as others have said it's a long job testing to prove OK, quicker if you find a fault. The volt stick is useless it would not help at all.