On Thu, 12 May 2011 00:26:55 -0700 (PDT), deano
wrote:

How can I go about finding the fault?
Depends what test equipment you have (DMM, Avo etc)?
You really do not want to be using a wet finger to fault-find.

It happens when the isolator
switch is on or off! I'm wondering if the unit is faulty or has a
fixing nicked a cable in the wall?

Does the wiring from the isolator to the transformer (assumed in the
cabinet somewhere) include an earth wire?
If so, is it connected to the metalwork? Continuity?
I would have expected the isolator (FCU?) to disconnect the line from
the transformer so there should be no leakage possible.