Mmmm - with some assistance from my father-in-law (who's an electrician) I got the HU installed today but it is showing characteristics of not being grounded properly and/or this workaround not working.
There was a message that came up but the wife dismissed it so I've no idea what exactly is said but it was something about a pulse speed indicator. Also, a lot of the menus (e.g. "Firmware Update") were greyed out until I did a cold boot (power off/key out, power on to ACC). If I started the engine and tried to amend these settings, they were greyed out.
I'll take the unit out again tom