Dundonald wrote:


1. why didn't/doesn't the RCD trip before the fuse on the consumer unit

As Ellis said, an RCD only detects faults to earth, most obviously L to
E. If your wiring or lamp had an L-to-N short, that'll make the MCB pop,
but the RCD will be happy as Larry, since just as much current - in this
case, lots and oodles - is flowing out down the L as is coming back up
the N.
2. any ideas what the likely cause of the problem is with the lamp -
just out of interest? Like I say I'll just get a replacement from B&Q
anyway but just curious.

As mentioned above, a fault making the L shorted to N. Either a fault in
the lamp, or - more likely, I'm afraid - you wired it up wrong (or ended
up with bare L and N conductors touching when you sqaushed them back in
place). The blackening of the brick, and the popping of the MCB, says
you had quite a lot of electricity flowing for a short time. When big
fault currents like this flow, they can 'clear' the fault by burning
away the metal bits wot are touching where they shouldn't be - so
explaining your 'well now it doesn't trip the MCB any more!' observation.

HTH - Stefek