I suggest you test all circuits for insulation resistance, making sure to remove the neutrals from the consumer unit busbar when you test each circuit. Then find the fault on that circuit by breaking the circuit down into bits.
He did this, and I believe found that the spur to the garage was about 6 megohms I think. Does that sound the right sort of figure? - he said it was still 'legal' I wouldn't have thought that resistance would allow enough milliamps to pass would it? I put the garage back on today as I needed to use it and it only tripped the once, and then when I wasn't actually using anything (again!)
Re the meter. Would most electricians be expected to own one of these? He did have a big box which beeped a lot which he seemed to use for doing most things, but I didn't see anythng like the picture shown.
We've just had a new CU fitted and the electrician who fitted it is stumped.
It's a 15 way box with 4 lighting circuits and freezer running from the main 100mA RCD and the rest running from a 30mA RDC. It's been in a month and initially tripped several times a day or night with no pattern to things in use at the time.
He initially tested the RCD and found it to be faulty and replaced it - it now only trips every 3 or 4 days.
By leaving various circuits off we have fairly confidently identified the problem as being the downstairs power ring. He has identified that this circuit has a lower leakage resistance - still many megohms and legal, but less than the other circuits. I'm not sure that this was enough to cause enough leakage, but we are getting desperate for ideas! We have spent three hours disconnecting each downstairs socket and testing the wiring between each. One length in particular - a spur out to the garage has the lowest resistance so we confidently disconnected that. Today it tripped again, even with this disconnected.
At the time it tripped, nobody was in the house and the only things running were a fridge, satellite and Tivo box (like sky plus) and a couple of computers plus clock radios and such like.
Had a similar problem on a job recently, Turned out to be the central heating pump was going faulty and tripped out at random, Just lucky in the end the Gas board replaced it and all been good since, Sometimes it not necessary the circuit you think it is, RCBO's would eleiminate all other circuits and lead you to the faulty one, Mixed Neutrals can be a problem but normally a pattern would build up. Good luck