We returned to Brisbane to effect the repairs on the Mac.
We found that the disk drive was still OK so the fault was certainly in the mother board. Since we didn't have a replacement, we decided to swap the Classic for the old LCII which wasn't being used at all and had been consigned to the cupboard.
We installed the 100Mb drive from the old external drive box and installed a new system. We put the original 40Mb drive from the Classic into the external box and transferred Mel's data. We then put the original 500 Mb drive from the LCII in the external box, copied some useful files from that, and cleaned up the file structure to make life a bit easier for Mel.