All,
Thanks for your various thoughts and suggestions. To answer some of the questions:-
Useless ****: yep, it's 30mA
plugwash: yes, in fact I've managed to isolate it to a particular segment of one circuit and it is (spasmodically) reproducable (i.e. doesn't trip every time the light is switched off, perhaps every fourth of fifth time). When I completely disconnect the cable to the second light switch its stops tripping but if I just reconnect just the earth it starts tripping again?! The cable is brand new and I've checked each of the conductors for leakage with a megger...
Damocles: yes, all neutrals are definitely in the right bars. Don't think I've checked for crossed lives - are you thinking that a light might have been run off the ring main somewhere? I don't think this is the problem - but worth checking - thanks for the suggestion.
mapj1: yes, checked for N-E shorts already, but thanks all the same!
sitinduck: sorry to hear I'm not alone! The only advice I can offer, having actually solved the same problem with another light (!) is to consider replacing the cable. My cellar light was tripping the RCD as well. The cable was a little old and had no earth, so I changed it and it no longer trips. Unfotunately the other light that does still trip is wired with new cable, so clearly not the same problem!