Good news Everyone! The lights on the trottle come on, and the smart display powers up as well.
That's the confirmation that I was hoping for.
Hopefully it will remain like this for a very long time, but
(if you haven't already done so) please make sure that you safely insulate the four unterminated wires for the horn/lighting on the front control harness.
If the metal conductors inside those four wires are allowed to touch against each other, you will cause a short circuit across the battery wires by either pressing the horn button or operating the throttle switch, which could instantly melt some of the small wires inside the harnesses.
I think that the faulty soldered joint in the old controller's harness must have prevented this from happening, as it was basically a weak link and behaved like a resistor, this stopped any high current from flowing which in turn, prevented serious damage to the wiring harness.
Alan