This diverter is easy to implement using an arduino and an £8 dimmer
such as Dimmer module for 16/24A 600V High Load, 1 Channel, 3.3V/5V logic|Integrated Circuits| - AliExpress
The arduino is connected by a usb cable to an emonPi and receives the excess power
emon/emontx1/power1 over the serial usb link from node red running on the emonPi.
I have some addons that read the temperature of the tank so as not to overheat, and switch the heater on in the off-peak period in the night without regard to the excess solar power.
I have bonded on some extra heatsink to the dimmer.