"I believe if I keep EA high I will get memory-space 0000H-1FFFH internal and 2000H- external ?"
Yes, but only if you are using a derivative wirh 8k internal flash.

I hadn't realised that a 'const' would get moved as if it's a code-byte - so I just need PSEN then ?
Yes

PSEN allows me to access EXTFLASH as program store/'constant code' data.
yes, MOVC activates PSEN

(I'm amazed there seem to be no example schematics available anywhere utilising FLASH and RAM - maybe I'm looking in the wrong places).
Well, again this is a microcontroller, program (flash) code is not supposed to be changed but for code updates.

Again, you need flash CE whenever you access flash for whatever reason, this, so far, seems to be your main problem.

Erik