So the trick suggested by blackswan40 did it. Uninstalled, reinstalled in c:\games\sh3, patched, extracted the 5 files from rld-h14.rar and then applied the 4 GB fix.
I am able to run the game without any compatibility mode settings- so far. Have installed no mods of any kind and before I do took it for a cruise last night to see if it was stable. Works fine, although I get flashes of some artifacts in the sky all the time. I pulled up the Nvidia performance overlay and it is running at 750-820 fps...
The GPU is puttering along at around 25% and at 35-38 celcius.
I just need to re-learn all the keyboard shortcuts, I have my original layout sheet from the retail box but still managed to fire 3 torpedoes blindly when I didn't mean to by tapping the Enter key on the numerical keypad. Also so used to WASD for movement in FPS games...
I also took 2 shots at a freighter at under a mile and missed... because I couldn't remember how to operate the TDC to aim torpedoes.
I think I stopped playing in 2015 when I got addicted to World of Tanks. So I will attempt to update my skills again.