I'm now pondering whether or not to put a ground spike in and attach cross bonding from the water pipes onto it. That might just do the trick.
It might make it worse and seriously increase the danger. If the bath is in contact with a live conductor and the pipes are not earthed then the shock will be minor ( a tingle ) as the pipes cannot carry current to ground. If you then earth the pipes they will be able to carry current to ground and the tingles will become musch greater and possibly fatal shocks.
In your position I would bond the hot and cold pipes together and extend that bond to the bonding tag on the bath. This will bring bath and pipes to the same potential ( equipotential ) and remove the possibility of a shock from bath to pipe.
but that might cause other problems elsewhere in the house.
If the bath is in contact with a live conductor and there is no bonding of pipes to protective earth then the new bond will extend that false live into all the pipe work in the house. You would then get tingles or more dangerous shocks from other taps. So you need to find the source of the voltage difference that is causing the tingle.