That's very informative, I didn't realise the whole horizontal stabiliser itself can be a moving part - I always thought they were fixed and trim was done using the elevator surface. Thanks for the links.

So essentially, trim changes the relative 'zero point' of the elevator, and if past a certain angle, even if the pilot applied full nose-up elevator, the altered airflow over the smaller control surface would be unable to counter the altered airflow by the stabiliser itself having been adjusted. I now see why recovery is so dependent on quickly disabling the trim system and resetting the stabiliser to 'true zero'.