Just a little more exploration of the vehicle; I wanted to see if I could keep my magmount (temporary), amber beacon on when the vehicle is off, shut and locked. Now, the manual suggests that to flick the PWR switch on the overhead array will do just that, by turning the auxillary sockets on.
Now I tried my phone into the USB sockets and no, they were not on with the ignition on, (cue dark muttering about build mistakes or poor quality)
So I then tried my amber beacons in the 12V power socket in the centre cubby and the loadspace socket and yes, it works, even with the ignition off!
I have just had a hunt through the owners manual to see if it tells me what should work when the PWR switch is operated, but no, there is no mention of the roof switches at all.
The mini guide does detail the roof switches and says that "Power switch leaves 12V / USB power on when vehicle is switched off". So nearly true! Ineos need to decide what was intended and whether to change anything.