OK.... here you go.... PZX2 - on the title screen, press (in order) F3, F7, F5, F6 PZX1.5 - On the menu screen (the one with the hands) hold down keys J, E and T all at once.
Cheers! In the times of original Speccy, i used to hack 100s of games (i made a machine code software for that purpose, sitting in screen memory, with lots of features including even step execution of z80 code, to decode R-register encoded loaders of Bill Gilbert)
