2 year old split load MK CU with 30mA RCD, PME supply, lighting circuits
and outbuilding (with own CU/RCD) on non-protected side, everything else
on protected side, no history of tripping.

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.