@1estar - yep I was going to use the lower element for my solar diversion, and leave the middle element wired into my solar H2O controller since I only want to boost the top half of the tank at 4am, but want to store as much thermal energy when heating via PV diversion. But since I only have a single feed from my distribution board to the HWC I need to have some safety mechanism to ensure both elements can't be on at the same time - hence the SPDT relay.
@aredwood - you probably know more about this stuff than most but it might be worth checking out the energy monitors from OpenEnergyMonitor. They are relatively cheap, even when shipped from the UK, and the guys over there are pretty knowledgeable and willing to help. If you needed any help with the software side of things I would be happy to help if possible.