Update... so I sat down last night with my best glasses and a tiny jeweller's screwdriver, to unpick the +12V and +12 Switched pins (red & yellow) from the connector block on my spare loom, and swap them.
Then this morning, I got an email from Erisin Support:
"Please enter: settings--car--factory settings with code 3368--Base settings--Power off memory--turn it on and then restart the stereo."
Now it retains the stations, with no rewiring
Why wouldn't you have it set that way as standard? Oh well, it works now, which is great. However, I'd like (a) the radio to come on when I start the car, rather than just the home screen; (b) when I do switch the radio on, for it to start at the last selected station, rather than 87.50... maybe that's hidden in the system settings somewhere too, will have a play.
Another annoying thing about the unit is that it takes about 40 seconds after switching the ignition on to boot to the home screen... are all such machines like this?