sounds very much like a problem i got called out to once on a tns system, all they said at the time was if they touched the sink or rads without anything on their feet they were getting a shock but if they had their shoes on they was ok, so that's what they had been doing for 2 months, went to check it out, voltage present at sink and rads, same again no rcds tripping, problem was out in the street at the joint coming into the house, joint had blown sending voltage up through the old lead sheaf, electricity north west was out within 20 mins, and took them 2 days to sort it out after a lot of digging, just mention it to your electrician so he can check, will only take him 2 mins before he starts ripping your house apart