On Wed, 12 Mar 2014 08:04:00 -0000, "Brian Gaff"
wrote:

These weird effects seem to be quite common according to friends of mine,
even a nearby thunder storm seems to do this sort of thing, presumably by
induced current charging up a capacitor.
I'd have thought these lamps or fittings ought to have a very high
resistance resistor across any capacitor to stop it.
Brian

Thanks for all the replies; they have been very interesting.

It seemed curious to me that the flashes only occurred at night. I
wondered if perhaps they were happening all the time but were less
visible during the day. However I don't think that is the case. I did
wonder whether like one reply said, it is dependant on another circuit
being in a certain state nearby, so perhaps it only happens when
another light nearby is switched on?

However this morning it occurred to me that there is a fan over the
shower which is switched by the light. The fan has an isolator and
SWMBO switches off the isolator and only switches it on when she has a
shower, so possibly there is something amiss with the fan and it is
passing current from its permanent live to the switched live which is
causing the flashes. I will investigate more.

Thanks,
Stephen.