On Tue, 06 Aug 2013 09:58:03 +0100, Lobster
wrote:
Currently being driven nuts by the above... I have a split load CU. When
the RCD trips, it's only that which goes; ie none of the MCBs trips at
the same time which means there's not much for me to go on.

I has similar issues for years, which turned out to be a combination
of things:

1. An oven element, which had a leakage that was proportional to its
temperature, going up to around 10ma when it was red-hot. Measured by
breaking the earth wire out and measuring the current with a
multimeter on mA AC range. Cured by replacing the element.

2. An outside armoured cable with a low live/earth resistance when
wet. Discovered with a megger, cured by disconnecting the circuit.

3. A large number of switch-mode power supplies: household PCs,
router, NAS Etc, which all added up. Cured by replacing all the 32 amp
MCBs with RCBOs to give each ring it's own 30ma leakage budget.