Pokey - going back to post #199 about the 2 mystery wires, I asked Caterham also about this last year. They said it was used for racing 420R cars for the oil tank temperature sender, but was not required otherwise. I guess they have different ECU programs for different applications. But I had the spare sender (as you do) and so I installed it anyway in the hole near the bottom of the oil tank and attached one of the spades to that, for the second spade I drilled a hole in the bottom flare of the tank and bolted on a ground connector. I'll send a pic if you want. I ran the car with it connected and disconnected and couldn't tell any difference in operation, so they are probably right it's not used, but as OCD as I am it just felt better being connected. When I dug into the CANbus I couldn't find it there either, though I got bored somewhat after finding all the good stuff.
About idle - my 420R idles at 950-1000RPM, don't know if that's the best RPM for idle but it seems fine to me.