Andy Wade wrote:

Or perhaps something that got wet and has now dried out?

I realise that things could have changed during my reset attempts,
either by an appliance that I've not used since remaining on standby, or
the wind changing direction, or self heating/drying from the leak
itself, but the sequence of events was all within just a few seconds,
and it definitely wouldn't initially reset with the MCBs on, but then it
would reset with them off and didn't trip as they were all turned back on.

Sensitive "earth leakage" clamp meters like this
http://www.martindale-electric.co.uk/clamp.htm#cm67
are now being promoted as a good way of tracing faults causing RCDs to
trip.

Useful to know, but excessive to trace a single episode ...