Got my spare part out of evil bay yesterday, payed 20 GBP total.
For some years it was used as well for the flush fit mounted reverse light on Austin Minis but most of the parts specialists don't stock it any longer.
So get one now.
If you have some connections to the UK they may help you out.
Some links:
Ships UK only:
CLASSIC CAR WIPAC REVERSING LAMP NEW OLD STOCK | eBay
This seller ships world wide and you get two:
WIPAC reversing light caterham Lotus Mini Cooper Capri Escort Vespa Scooter Imp | eBay
Uwe