Thank you for the responses.
Turned out to be just my joystick - tested it by unplugging in flight, and tapping (rather than holding down) the 0 key put it up to 1800 rpm where it stayed. I guess holding down the 0 key had the effect of over-riding the throttle input from the joystick, which just needs to be recalibrated on the throttle axis.
I guess I should have tested that first! But I will wait until I am out of training before I re-calibrate it, as it is such a cool (if accidental, on my part) feature
I can understand why these things are not fully modelled, as returning after a fairly long absence I can confirm that the learning curve for a flight sim like this is hard enough without introducing lots of fiddly engine management aspects.