The solution, after a lot of googling is this. A microswitch is fixed to the bottom of the point motor. The COM tab is wired to the +ve feed of an ancient Triang uncontrolled DC power supply. The OFF tab is wired to one leg (anode) of a bicolour 3 legged red/green LED via a resistor, and back to the -ve terminal via the common cathode (centre leg). The ON tab is wired to the other leg (anode). This means that this LED will light green when the point is set "off" or normal, and red when switched. An identical LED is wired in the same way, in parallel, but the opposite way round, so it shows red when the point is set "off" or normal, and green when switched.
Here's a diagram:
And here are photos of the test wiring:
I initially tried wiring with two-legged bicolour LEDs, but to get the desired effect requires extra diodes and other electronics stuff - so it's simpler to use the 3-legged versions. I used Maplins for the test but they are cheaper at All Components and Rapid.
I used a cheap microswitch (50p at Rapid, £2 at Maplins!) which will be glued to the bottom of each Peco PL10 motor. You could use Peco PL13s but they have a bad press, and they are much more expensive (£2.80-ish). Here's a photo:
ian Lambert's site.