Update.
Got the loudlink.
A bugger to get the changer out even with the supposedly correct tools. Which needed finessing with a file to go into the release holes. A local "the man" I use for other things had the knack. Then wouldn't come all of the way out.
Had to take the under-the-compartment/above-the-pedals trim off to snip a cable tie which was restricting the cable and out it came.
Followed the loudlink instructions.
Threaded the microphone between screen and trim. Only visible cable is with the drivers door open.
Easy to "fit" but somewhat fiddly to setup for my portable 'phone.
Mapping the existing headunit user interface of CD & track selection to the Bluetooth pairing, music playing and phone processes is clever but somewhat non-intuitive & necessarily limited.
I'll get used to it I expect.
I haven't got an SD card (yet) nor have I used the USB charging hole.