Eventually I settled on Karabiner Elements, an app that lets you create a simple modification specifically to target the Mac internal keyboard where you can map `non_us_backslash` (that strange unused key) to `grave_accent_and_tilde` which fixes this.
Back on Mac now with British PC and it's much less frustrating to use the keyboard. Plenty of other frustrations instead. I'd prefer to be on Linux but the hardware is just too good.