Ok, the story so far ...
I've tried the following methods (on white, cast acrylic, 3mm thick):
- Pressing at 190 °C for 60 secs and cooling under a weight - Result: Image great, but mild distortion of acrylic.
- As above, but then re-heating to 100 °C in my halogen oven and pressing COLD for 30mins - Result: Same kind of distortion and SHRINKAGE.
- Pressing at 170 °C for 30 secs and cooling under a weight - Result: Image faded around edges, still some distortion.
- Finally, pressing at 190 °C for 60 secs and just allowing to cool naturally on a flat surface - Result: BINGO! Great print, no distortion, no shrinkage!
So it appears that the best method is to let the pressed acrylic settle naturally on a flat surface with no added weights.
Still early days though and still experimenting.