Frankoo;
It
is possible to change the switch with gearbox/OD in-situ, but I can think of a lot of other more funner things to do instead.
You state: "...it is definitely the switch!!! Crawled underneath & messed about with the wiring to see what happened....." I'll believe you if you insist...but this does not tell me
how you verified switch was the problem...switches typically fail
open and so are incapable of energizing control relay...so I have my doubts! I'd verify switch function in the engine compartment by having someone watch reverse lights as I pulled the Black wire from terminal 85 of the Revering light control relay (with IGN ON reference wiring diagram:
http://sw-em.com/122S%20Wiring%20Diagram.jpg), If switch at gearbox is indeed shorted and calling for the relay to be activated continuously, this will verify the problem condition.
If this is indeed verified, I'd second your Plan B...pull the wire and run it to (or temporarilly under) the dashboard where I would install a manual grounding switch. That would allow you to put off the repair until next time gearbox is out and you have better access.
Cheers