Maybe it is not the bath that is at fault but whatever is around it. I know what follows is a long shot, but it's the only I can come up with at the moment:
On the circuit, there is some equipment (towel rail or washing machine, maybe?) with capacitive coupling to earth or poor insulation leading to a slight current trying to find its way back via the circuit protective conductor. However, the cpc of the circuit has somehow been broken. This means that the equipment and possible the surrounding area are live with respect to properly bonded objects, like the bath. Thus the tingle.
When the bath is plugged into the circuit, the circuit becomes bonded/earthed through the cord to the bath. This eliminates the potential between the bath and whatever is connected on the circuit.
{Edit for lack of grammar}
[This message has been edited by C-H (edited 01-21-2005).]
