Any opinions on the likely cause?
Cocked up STUPRECC drill with a simulated HYD fail? Use an excessive nose up TPI value and you'd probably run out of 'unlock' elevators to maintain level flight as speed increased? In the early days, the Gnat didn't have a feel trim gauge, so setting the correct tailplane and feel trim position values would have been much more difficult - you had about 30 sec before the TPI froze, after which you could only move it nose-up from the failure point using standby trim.
Slow down until the stick was 'load free central', reselect the HYD power cock ON, relock the elevators and it should be OK?
Please would a Gnat QFI comment - I was a mere struggling student! But I'd been advised that it was vital to understand the longitudinal control system fully and to know the STUPRECC drill so that you could recite it word perfect at any time of day or night. Screw it up and at best you'd have to eject......