Last week I noticed I could hear the normal high pitched tone coming from the Throttle Body, but this was about 5 hours after the engine shutdown.
Inserted key fob and got the HDC Not Available and no cranking when attempting to start engine.

Stripped out the throttle body and all appeared ok, gears all in good condition and open/close of valve worked ok. Reinserted and eventually got engine fired up. It seemed to take a bit of judder (ie roll the car a couple of feet) before the I could get the engine started ( I began to suspect starter motor was at fault).
It ran ok for a couple of days, plenty of stop/starts, but the high pitch tone did seem to vary after engine shutdown, sometime the normal 30 seconds, often for few minutes and occasionally never (even after double locking of car). If the tone is continuous then then engine will not start. On at least 5 occasions, giving the car a push and a 'judder' has then meant a successful start!

I took out the throttle body again, but left the main control cable connected. I could see the signal (the high pitched tone) from the ECU was controlling the motor to open/close the valve, but continually trying to do this even when the valve was in the fully open position. As detailed by alex_pescaru elsewhere on the forum, there is a potentiometer which indicates back to the ECU the position of the valve. It is this that appears to be the cause of the problem. If I trick (by shorting two of the pins) the potentiometer to signal back to the ECU that valve is open, then a short close/open of the valve is performed and the the high pitched tone will stop.

This throttle body was replaced about 5 years ago, what I can't work out is what has changed to cause the fault - possibly the position of the copper brushes on the potentiometer because the brushes have worn and the resistance changed??

J.