On 03/03/2011 08:44, Andrew Mawson wrote:
I've isolated each individual circuit, and checked with a
megger to earth for leakage and there is zilch.

Did that include checking N to E? The quickest test is to isolate the
whole installation, strap L&N and megger that to earth.

Otherwise hire/beg/borrow/buy/etc. a 'leakage clamp meter'. I bought a
Fluke 360 a year of so ago and have found it extremely useful, for
general diagnostic work as well as leakage problems.

--
Andy