PaulC,
Shouldn't the Armouring in the SWA cable be at Earth potential?.
I would have thought that this would tend to drain away any Capacitive effects from within the cable.
Is the Cold water main actually buried before it enters the house, if it is, that should also be at Earth potential.
Are the hot and cold water systems bonded together?, I have seen Elements leak current into the pipework, before today and it is usually the sign of an imminent element failure, however the current is too low to trip any RCD protection.
Only other thing I can come up with is:
Is the house on one single RCD or a couple of them?.
One thing I have come across before is where an "RCD neutral" was connected to the wrong Neutral busbar in the switchboard, referencing it back to the Mains earth, this in turn, causes tingles at poorly earthed fittings.
Other than that, I'm empty!. [Linked Image]