I have been using a FAT16 card for flashing - this was'nt exactly the cause of the problem, but having viewed another post on the OZ forum, I managed to solve this.
What I need to do was download an old Sharp ROM - but instead of naming it 'Ospack', it was renamed to 'ROMIMAGE'. I then ran the flash update from the service menu, which allowed me to flash the sharp rom this time. After it finished, it crashed out into a shell again.
However, it must have recreated the original partitions, because I was then able to reflash with OZ or tCKRom, and this time it booted up OK.