Reading through this whole thread - well I'm shocked to say the least (pun not intended), failing to find the cause of a potentially life threatening voltage within the confines of the bathroom, in particular the shower, the solution taken was to somehow earth the wall but notely not removing the initial problem.. who-ever thought this was a safe solution is incompetent and I would question his actual actions and his ability to do his job safely.
The Electrician(s) who have tried to find this fault may be inexperience, the fault may be elusive even for a seasoned Electrician but however you look at it, the safety of the customer is priority and regardless of the front end device been an RCD it shouldn't be seen that this could not kill someone, the 30mA trip value is a compromise between having nuisance tripping for designed leakage and giving some form of safety, getting a shock under its trip threshold is no guarantee you will survive albiet the risk is lower but considering this is in a shower where your body resistance is reduced dramatically as your skin is your main protection against voltage, it cannot give this protection when soaked ,thus there is every possiblity that enough mA could cross the heart and cause it to go into fibrillation - its a very complicated calculation as so many variables exist but knowing my job, I wouldn't play Russion Roulette in these circumstances.
As the situation stands, I would not be using the shower under any circumstances, until the cause is known and steps have been taken to stop it rather than by-pass the issue.
I would consider an Electrical Engineer if you have exhausted your time with local Electricians with no results, they will have more indepth training and be accustomed to diagnosing very tricky to find faults - this should be a breeze in the park for them.
I would be thinking outside the box here if the general tests have come up clear and haven't identified any particular circuit.
-Other considerations would be - Aerial booster, TV fault making the Aerial co-ax live and could hold a lengthy charge due the electronics even after power is removed.
-The voltage could be from an external issue and using your Bonding to route to a good earth, this was an issue I had in Leeds once where the DNO joint in the road had earthing problems and an intermittent fault next door used the plumbing in the house I was in to return to earth - a Clerk of works nearly lost his job over this fault as he wouldn't believe my theory and it turned out to be the neighbours microwave, I confirmed this to him by pullin the DNO fuse for safety but the voltage still kept appearing.
-Internal fault less than the trip threshold and could be down to an isolated section of pipe which is either giving a floating voltage on the wall or the shower itself has part of it isolated with a floating voltage on it.
What ever the cause I can only say - DO NOT USE YOUR SHOWER! ... until this is resolved.
