Matt Prescott calls to say the E-Day web site is live.
Energy saved will not be apparent until 6pm, when the switch-off starts, he says.
update: BBC online story here at 2.51pm
P.S. checking in at 8.45pm, consumption today actually seems to be running at some 2,000 MWh higher than "normal": a negative saving (i.e. excess) of 1.7%