I've just been out experimenting.
I disconnected the plug from the rotary coupler and hot wired a switch across the light green/red and the black wires.
It still doesn't work, so the fault is obviously elsewhere.
I didn't realise when I was checking at Gaydon, but there are actually
two fuses for cruise control. I've checked the other one and that is also good.
A couple of things are confusing me. The wiring schematic shows a cruise control ECU for the petrol model and a cruise control relay for diesels. Mine is a diesel. I can find no mention of this relay - R142 - or its location.
The schematic also shows a feed from fuse 4 in the engine bay fuse box to the CC relay.
Last but not least, the schematic for the rotary coupler would appear to have an error. I would expect the common side of the two cruise switches to be connected to ground, along with the horn buttons, but the schematic shows them connected to the wrong side of the horn buttons.
So, does anyone know where the CC relay is located? That might be my next step, along with checking fuse 4 in the engine bay fuse box.