Further to my initial post, the Geyserwise tech chappie did call me. Apparently the alternating EA and E7 means software corruption. They can try fix it, but generally a new controller is indicated. Seems like a hardware upgrade to CMOS of something similar could solve what seems to be an increasing problem. Apparently the surges resulting from Eskoms load shedding are not good for the board.
So I went to Centurion today and the guy who helped me said E7 is the problem indicator. Turned out I needed a new controller. Surprise.
After installation, system still not working. Now getting E3 which is geyser sensor not found/working. I have checked it all the way back to the control unit. I get just over 4kOhm which seems in range for the sensor.
If I swop the two sensors around (solar panel temp & geyser) I confirm that the input to the controller is working. I'm stumped and would appreciate ideas.