Little update on this thread, as I raised issues with my Alpine unit.
I got in touch with Alpine, and we went through some checks and diagnostics. Having already sent the unit back to Alpine, they agreed to swap the unit and the Canbus interface for new ones. I put this in about a week ago, and the unit has behaved since then. Much better, and the sat nav hasnt gone adrift since then. The MFD display no longer shows odd messages, and no longer has a little error in the corner.
So, it is now a thumbs up on the unit, and Alpine were very helpful with replacing the unit. Had to give them a deposit, but they sent me a new unit upfront so that I could swap it over rather than leaving myself without a unit for a few days.
Obviosly it still doesnt pinch to zoom, and sat nav mute is buried in the menus, but I normally use carplay for everything, so that all works good.