Installed driving lights (finally)
I should close the loop on my original question.
I did buy the genuine LR momentary switch. I then made up a (complex ...) circuit that toggled a relay which connected a line from the high beam switch to the main relay behind the RF headlight and to the switch LED. The main relay conventionally switches power from the battery to the driving lights.
One point that I don't think has been made is that electrical safety/protection means that the power should NEVER come straight from the battery. Evenwith a fused relay, there is no protection against the wire from the battery to the fuse chafing and shorting - which would result in the sort of nasty under bonnet smoke, heat and smell I noticed coming from a parked Lexus the other day (called the fire brigade and the car was still there hours later ...). I took my power from a 150A fusible link in the engine bay fuse box. That way a major short should be arrested before further damage.
Many thanks to PaulMc0308 for the switch diagrams - it was very hard to puzzle out with a multimeter.
What I have now is a genuine LR driving light switch that activates driving lights only when high beams are on, and needs to be re-activated after headlights have been dimmed or switched off. That way I won't accidentally flash people with the driving lights when I forget they're on and hit the high beams when a bit too close for the driving lights ... I believe that's the original Land Rover intent.
The circuitry is hardly genuine though ...