Just had the RCD trip out, went down to investigate, it tripped straight out again, twice, so initially appeared to be a fault remaining somewhere. To identify which circuit I switched off all MCBs on protected side, the RCD then reset OK, so switched on MCBs one at a time expecting RCD to trip when I reached the offending circuit, but they all came back OK.
Of course now I wish I'd done it the other way round, switching *off* MCBs one at a time attempting RCD reset after each. Still seems strange behaviour though, will see if it re-occurs, any possible explanations for the sequence of events?
Luckily I was at home when it happened, if nothing else this serves as a reminder that I really ought to have a non-protected circuit using a spare way on the CU for fridge/freezer (perhaps central heating too) and to fit a rechargeable battery back in the radio alarm clock.