Ausminiac you will need the following:
Reversing light sw - Part No AYB9194 (Lucas sw No34887)
Steel Plunger Part No 22G351 or AYB9195 - 3/16"dia, 0.515" long (33/64")
Lucas Australia 2 screw rear lamp bases with reversing light sockets - scarcer than hen's teeth. You can probably modify your existing ones by replacing the indicator socket with a twin filament stop/tail light socket from another base. Haven't tried it and you may end up with 2 stuffed bases.
An alternative is to wire up the reversing switch to the rear indicators with a pair of diodes at each rear indicator so that when the reversing lights are on the front indicators don't come on and when the indicators are on only RH or LH indicators flash and you don't get feed back to the opposite side through the reversing light wiring.
Electronic whiz needed here!
Remove reverse detent plug from front of gearbox. Be aware there is a big spring behind plug. Fit steel rod down through centre of spring. Screw in reversing sw in place of plug. Use new washer. Run wiring from "Ignition ON" A4 fuse to switch to rear indicators. It may be possible to wire in the diodes under the dash so you don't have to run the wiring all the way to the rear of the car. There is a junction between the front and rear wiring looms either under the dash or behind the LH trim. Not sure exactly where. Can't remember. KRAFT disease.
The Lucas switch number is quite old (30+ years) and may no longer be current and you can easily make a plunger from a piece of 3/16" dia steel rod.
Hope this gives you a start.
RonR