On Tue, 06 Aug 2013 09:58:03 +0100, Lobster wrote:

Question 1: if, for diagnostic purposes, I switch off each circuit in
turn by manually flipping its MCB, does that actually isolate the
circuits for the purpose of elimination?

Sort of. If the trips are being caused by a live earth leak then
switching off the MCB will give an indication that that circuit has a
problem. If the trips are from a neutral earth leak then switcging
off the MCB won't make any difference.

Question 2: what does the fact that no MCBs are tripping; and the trips
occur at apparently random intervals (4-5 times in the last day, with
interval of up to ~10 hours so far, and including an overnight trip; and
no problem with reinstating the RCD immediately) tell me about the
nature of the fault, if anything?

It's intermittent. B-) Others have pointed out likely appliances
but it could be down to rodent nibbled wiring...

--
Cheers
Dave.