Bazz: "Another catch is the connection to the mains might be such that at 12 volt it does not make a solid connection but at 240 volt it breaks down any dirt or grease etc."

You would use a mega-ohm meter, surely.

That aside, it gets worse. The favoured bats have metal foil on one side, fibre on the other. The metal side is laid face down ...

Anyway, there is an obvious technical fix. Just insist the entire sheet is electrically connected, and then earth the lot. Testing something is electrically connected is easy and should be relatively reliable.

Rstuart;
Yes, of course, it had slipped my mind, I haven't had one for
at least 40 years.