Ok, so I finally managed to export successfully from Godot 3.2 stable using the below:
iMac : Sierra
xCode : 9.2
iPhone6 - iOS : 12.3.1
iPhone7 - iOS : 11.3.1
Noting that the default /stable distribution of Godot 3.2 can export to iOs armv8 only (there are workarounds for armv7) so a minimum of iPhone6 is required.