NOTE: Just something here for anybody running this on the latest core/NextZXOS 2.07k... While playing it on the physical hardware, occasionally the background of the text on the title screen went purple after entering the initials into the high score table...
Why? No idea as there are no purple colours used in the game (for printing) at all. Re-running the game seemed to fix it, but I've heard someone say it happened a few times. It's more a visual aesthetic issue on the title screen and doesn't detract from the game.