SomeOrOther

Thanks
Torqued to spec but have not done a load test for hotspots yet.  Labelling on its way (read nicely cut vinyl).

Finally finished the installation and all is working as expected  - no floating voltages, tripping ELs etc.
Many thanks to @iiznh for your input.

I used a 40A Relay as the Earth / Neutral bond.
I also used 40A breakers and 10mm2 wiring to and from my inverter.
My reasoning for using a 40A relay is that in the case a fault the full 40A can be routed to Earth/Neutral and the relay/contactor must not weld itself shut. Anything more than 40A my breakers will disconnect the connection.
 

Thank you once more for your insights.
Your second paragraph made me realize that I was powering the contactor coil with live before E/L, and neutral after E/L.  It is now set to take power from grid before the non-essential E/L.
Will definitely do the earth spike once I do the panels (which is soon).
I'll have a look at the Sunsynk manuals as well for cross-reference.

Looks good, very similar to my wiring. I have a heatpump so I moved my whole house to the essential loads which made my wiring very simple. 
I believe the The Non-essential loads Neutral busbar that is after the RCD connected to "Geyser" and "GPlugs" must be kept separate from the essential circuits neutral bar. When in islanding mode (loadshedding) then the inverter should disconnect both grid lines completely. This will ensure that the RCD does not trip when switching between and from islanding mode.
It goes without saying that all earths are always connected and shared between essentials and non-essentials. You should also consider a copper rod earthed and connected to your earth should there be any issues when cables are stolen and earth/neutral might be floating.
The sunsynk installation manual does have some good diagrams ( I used those when I did the wiring for my install)
For 40A breakers you should use al least  10mm2 (This is what I used for my 8KW inverter)
For 64A you need 16mm2
I believe there are some SANS cable rating tables on the internet for AC wire sizes.

Disclaimer: I am not an electrician
The hybrid inverter should be wired directly to mains that has not gone though a RCD (but after the main breaker). You do not want to RCD to trip and the inverter taking over the load supplying power where power should be cut.
@SomeOrOtherLook at the PDF in this post