Thanks for the pointers Tony but I'm on top of most of that stuff
Hopefully the following may help anyone still struggling...
I'm not sure what was going on but I decided to do some tests enabling and disabling SMP and various options. I was more worried about the odd look of the "postage stamp" (dinner plate was the wrong expression as it's rectangular, not circular) effect of the overcast when viewed from above. FPS wasn't too bad for me.
For anyone struggling this is what I did just now which seems to have sorted multiple issues / niggles...
I rebooted gizmo which see