I work in the control room of a large power station. Over the weekend I was in over four nights. On each of these nights we saw grid frequency dropping suddenly at exactly 00:30 hrs. This is the first time our shift's noticed this. As all of us in the control room have EVs, we guessed what was happening and called National Grid to see what they knew. They confirmed the drop was due to the tariff change for suppliers such as Octopus Go. The frequency drop happens because of an instantaneous increase in power demand which is typically 500 - 600 MW. Right now, the UK electricity demand is just under 35 GW and, at 00:30 hrs it is around 27 GW (G. B. National Grid status) so the step change is around 2% this may not sound much but it is significant and has to be carefully managed by the Grid.