The message
from "Sparks" contains these words:

That is what I was thinking, I assume the earth supplied by the power
company is actually connected to planet earth at the substation, this, I
assume, is quite a way away from the house, so some current is flowing to
planet earth via the shorter route. So the solution is to install an earth
rod near the tub?

That'd depend on which earth system is in use. In our house there's a
tag beside the incoming cable which says which sort of earth it's got.
There are several.

--
Skipweasel
Pay no attention to that man behind the curtain.