The only way to "drag voltage down" is to draw a lot of current.
I'm surprised that so much could be drawn on a lighting circuit as to make incandescent lamps visibly dimmer and CFLs to fail to strike at all without tripping the MCB or starting a fire....
I have powered the consumer unit down and back and this seems to stop the problem for a few goes.
and
I have checked the voltage drop from live on the MCB to the neutral rail on the consumer unit and its giving me 244v when the lights are all off rising to 262v when all the lights are switched on.
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