Thanks to Arno for another suggestion that had not occurred to me.

I removed the idle valve and checked it. It was clean and the mechanism was free to move. I checked the resistance and it was 8 ohms, as stated in the Haynes manual.

I am not sure how this device works. I appreciate that the valve rotates through about 20 degrees and opens a gap that allows more air into the manifold, but my valve in the rest position (off the car and no power supplied) had a narrow gap along one edge. In order to open, which happens at the other edge, this gap would have to close thus shutting off the air totally for a moment in the move to a more open condition. Are these valves set up so a little air can pass in the rest position - there was a sealed grub-screw acting as an end stop which created this gap.

I observed the operation of the valve on the car by disconnecting the air inlet and looking at the mechanism through the inlet port. It did not move very much, but it did affect the running of the engine. At no time was it anywhere near fully open, I would think it only moved about 5 degrees is this what I should expect?