Haha. Well, incidentally, I have a bit of an update...
So it looks like that previous post of mine in this thread was the catalyst, and as I was driving around some time before Christmas the Android unit went up in smoke - literally.
I ran with a standard radio with aux and bluetooth for calls for a while until I started to miss CarPlay.
I was absolutely convinced that I wasn't going to spend £700-£1000 for the Alpine specific fit headunit, as IMO that pricing is ridiculous.
I ended up with a Sony XAV-AX3005DB and my total costs for fitting are:
Sony XAV-AX3005DB -> £295
Enfig RMK-E46-XAV -> $49.99 (£39)
Connects2 CTSONYLEAD -> £2.99
(I was able to re-use the 17-Pin -> ISO wiring harness and SWC from a previous Alpine single DIN radio, I just needed the fitting kit and Sony compatible patch lead.)
I spent a little extra to get complete functionality of the unit:
Enfig MIC-BM1 -> $29.99 (£23.50)
Eightwood EWAN146UK -> £28.99
So my total project cost is £389 for a fully operational double DIN headunit with reverse camera, carplay, android auto, DAB etc.
I still wonder how Alpine can justify their cost.