Wow - thanks Maverik.
For ease of wiring it would be better to run the warning light from the switch side of the circuit, rather than the light side. Hopefully there is no issue with that as it would save another wire running the full length of the Landy. Should be OK, as that must be how an illuminated switch works?
I just googled 'land rover defender diode' and bizarrely it found a post from Ralph:
Quite expensive for what it is ? If I can confirm its the one behind the warning lights it might serve to provide a rating.