That makes it much clearer for me, I was forgetting the difference
between my bulb test and a real world short !

Just to satisfy my remaining curiosity, I take it an RCD only trips
when power from it's own circuit goes to earth (as opposed to my test
which was live from the lighting circuit to socket earth).

Thanks a lot for the advice. I'll sleep much better at night with a
clear picture of how it all works :-)

Declan.