One step ahead of me again Mick!
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So...
... In a bold and reckless move, I've had an idea and tried it.
The cable I have bought for the other 308 has the right connectors and the right pins in those connectors used (you can see 4 empty pin locations in the rear of the head unit photo above. The same four are empty in said cable) so

- I've docked the small (new) connector in the radio's existing empty spot in its wiring plug
And switched it on!
I have had a whirrring noise and pushing "LOAD" makes the lights flash:

- Flashing
Radio does not recognise it's new friend. New device does not appear up for ingesting a CD.
Diagbox session next in the cards I think, but probably next weekend task.