Thanks for all the help.
I have looked again at the distribution box. It has two switches labelled main. One red 100A which switches everything off and one RCD which only covers the three CBs for the sockets(ground, first floor and garage) There are four CBs which are not affected by the RCD - Cooker, some kitchen sockets both 32A, ground and first floor lights both 6A.
The lights did indeed stay on when the trip went. Since the sparks who I will get is the one who installed it all when the extension was built I am guessing he will say he designed it correctly!
srobarts