the waste is plastic,, it discharges into a normal grid and doesnt even touch it.
all neutrals are tight,
the only soultion i could think of is there is a supply company pole a few yards from the house which has an earth stake,(pme) i have asked them to check it out just in case there is current flowing down here into the ground, then when you empty the bath you will get a connection to earth via the person pulling the plug and the water running into the drain.. the bath is enammeld so probably no earth connection through that,
it is very worrying,
