I sometimes turn up old threads in a Google search - a question is asked, there's a bit of discussion, but the OP never comes back with the final resolution.
In that spirit: Called in the electricians, who tested the circuits with a multimeter [Hmm. Could have done that myself. I had asked the dispatcher if they had a megger before they came out; guess he didn't understand the question, but said "yes" anyway.], found nothing and replaced the RCD. The tripping continued.
Went on for about 2 months in total, once tripping 10 times in a day, once going 20 days without tripping (20 days, mind, with the fault still unresolved, making fault-finding by exclusion well nigh impossible).
Then, happily, the fault became permanent and exclusion became trivial. (Although this happened while I was away on holiday; my neighbour glumly reported that they couldn't reset the RCD, so out went the contents of the fridge.) The problem lay with a PC monitor, and on further examination there was a live-earth fault in the IEC lead. Which had signs of apparent corrosion and even with a DC multimeter showed a live-earth resistance of 14K-ish.
Interestingly/sadly, this fault would have shown when the electricians tested the circuit insulation resistance if we hadn't first gone round turning off the IT equipment at the socket so it wasn't disrupted by throwing the breakers!
Hadn't realized how much intermittently losing my PC, washing machine, fridge, heating, etc. with no idea of how much it was going to cost to resolve had been wearing me down until I fixed it last night and went to bed a happy man. Woke up 8 hours later to my cordless phone beeping about lost basestation and no display on the clock-radio. Totally crestfallen until I saw the whole neighbourhood had lost power (back now!). What are the odds?