Update - I had another hour with the Clear Box this weekend, but didn't get much further.

1) Booted with the CF card and SSD containing the installed image - flashing cursor with no activity. No GRUB menu on escape key from power on (presumably if GRUB was fully installed on the SSD this should have come up).
2) Changed BIOS boot order to CF expecting to see the installation screen - booted to GRUB rescue command line
3) Wiped SSD partitions on other PC in hope that CF would view as new install - same GRUB rescue command line
4) Removed SSD to leave only CF card - GRUB rescue command line with error about missing volume.

AFAIK I haven't changed any of the files on the CF card, therefore I expected it to behave like an installation CD, and boot from this (as it presumably did on the first boot).

If anyone could provide me with some pointers / things to try it would be much appreciated.