On a similar thread, I went to a job today where a customer had received an electric shock whilst in the shower. Its purely gravity fed, no electricity involved, but the mixer is metal and its fed by surface mount copper pipes that run up into the loft where they join plastic. As the water main is plastic there is no bonding. When he turned off the shower he received a shock and the rcd tripped. So my theory is some appliance in the house is introducing the current into the water, and because there is no bonding the rcd didn't trip until a naked man standing in water earthed it out. The fault wasn't present on arrival so I've temporarily bonded the pipes to the nearest socket, going back tomorrow to get involved. . . What I think I'm saying is, if in doubt bond it, then charge for the pleasure, everybody's happy!