Sorry, I thought this needed a on of the Fruitcake ROM cards. RTFM...
The patch checked for the standard game keys (SHIFT and Z) but if neither were pressed then it checked for the 5 and 8 cursor keys. When using EightyOne, the arrow keys on the PC keyboard map to SHIFT+5 and SHIFT+8. Therefore pressing either of these will cause SHIFT to be pressed. Because the game code checks for SHIFT first, it interprets both arrow keys as a request to move to the left!
A possible solution is to change how EightyOne handles the arrow keys, changing the interpretation from "5,6,7,8 (Shifted)" to "5,6,7,8" under Options > Keyboard.
However, a more robust solution is to simply reverse the order in which the game checks for the keys, i.e. if it does not detect keys 5 or 8 being pressed then it goes on to check for the SHIFT and Z keys. The attached version uses this approach.
This version is perfect, thanks!
Attached is the .col file that I have declared to be the final version. Furthermore I have attached a standalone .txz file so that non-Chroma-owners can try the patched and colourized game in the EightyOne emulator, you just need to activate Chroma colour (menu "Options" - "Hardware" - change "Colour" to Chroma).
[EDIT 2018-02-14: attachments removed, see next posting]
The colour enhanced game can be downloaded from here.
For those without a Chroma interface, the game can be played using a compatible emulator. Emulators are available for a variety of platforms and are listed here.
ZeXtender board, Joy81 - Programmable Joystick Controller, Turbo Sound 81